With M810 calipers, XTR is a definite higher price point that will give you a one-piece formed full alloy lever body with a more "solid"/smooth lever action, also has adjustments such as tooless reach adjustment (same as SLX) + freestroke adjustment (not really useful). Overall a bit more "quality" parts, but doubt any reduction in praticality/base performance. Weight is negligible. I believe XTR will push a bit more fluid as the body is longer. Note, master lever reservoir push to a smaller slave piston area = more surface area power. The XTR body is definitely more "open" as far as dirt/whatever flying in. So I did cover up the underside and other areas to "protect" piston/seal area better.

As far as power goes, XTR is normally made with a 2-piston caliper vs. Saint 4-piston. XTR's full system power is rated @ 112nm vs. SLX 110nm-111nm. Power-wise, I don't think too much difference, except for overall feel and some difference in stiffness. XTR does have a open expandable bladder that expands during heat to compensate..this can be a good or bad. In the beginning, some people did have a problem with it, with me, nothing but solid performance.

Also, there is not going to be a HUGE power gain to switch to XTR/SLX. XTR will give out a bit more, but probably not worth it or hindering in anyway or big difference vs. XTR. The biggest difference is smaller/better lever feel, new servowave + bleeding. Setup is easy, just use M810's lines as BH80, or upgrade to BH-90 (saint lines are bigger vs. XTR BH90), but simple swap and connect.

 And I wouldn't call it frankensteining. All Shimano stuff is usually compatible and made to be swapped around. It's definitely the smart thing IF you get a good deal. Bit of extra brake power, better lever feel. Very easy to do.

Lastly, I would say ZEE or M820 Saint (-- Most ideal) is a better deal/performance gain. Only reason XTR was being put on because it was the only lever available right before Saint M820 came out. Reason why I say those, cause Zee is made for a dual-piston and higher output, while Saint M820 will be even more powerful (as that's the highest push-flow). I love my XTR levers, but on a standpoint of even more power, I would of gone with M820 levers (good deal of course). I would discourage against buying MSRP or new on-site unless a very good price. Overall SaintM810's are super super solid.

*My speculation on heat-control, M810 caliper defo, 100% has more material and was made for high-heat operating. Heatsink pads do pull away heat but I've somewhat researched and feel that airflow is most important. I don't think airflow is as good as it should be as if you look on any bike, the airflow is relatively blocked. Was gonna make heatsink pads for M810, but now going to focus on directing more airflow in form of air-scoops

 Sorry for long reply...I don't like just saying an answer unless given all of the info. TLDR: Not much difference, one will be more streamlined/higher quality but not much difference in actual performance or power. In the end, ZEE or M820 levers will be more suiting to M810 caliper.

 Cool thanks for the long reply. i appreciate it, better to give too much info than not enough!

Hopefully this isnt too long, but ive always been an avid guy and my buddy bought a jakal from my other buddy about 3 months ago, its got new slx brakes on it, and it has MUCH better modulation than my old style (non taper bore) code. although it fades alot faster than my brake (its got a 140mm 'wave light' rotor on it, lol) and doesnt as much power, ive figured that the saint could out perform the code just by how impressed ive been by the slx brake hes got. the gravity fed bleed process and mineral oil that shimano uses seems like a huge selling point to me.. ive always had to bleed more code brake more than once (3 seems to be magic number), to get them to feel 'okay' and still have a fair amount of lever throw (i run the red adjuster knob all the way out to help with lever throw), even with the help of a few friends who know what theyre doing (they also have issues bleeding their elixirs sometimes)..

soo, getting to the point.. as you may or may not remember, im building a kona operator currently (1st dh bike after 8 years of djs and bmx), and i think im going to make the leap and switch to shimano.. but ive noticed that some 'dh' brakes start at $450 and go up from there (new).. in an effort to keep costs down (the main reason why im doing this), i wanted to run the slx or xtr lever with the new saint calipers. seeing as how some people prefer xtr/xt/slx levers over the saint levers it just seems like a win win for me (cheaper and 'better') to buy slx levers and saint calipers..

 .. the one thing that i find really weird about this whole situation though is that people prefer the other levers over the saint ones.. shimano spends a lotta money on their saint lineup, you would think this wouldnt happen.. lol

 Yeah it all depends what kind of ppl you run into...I find most ppl I know like the Saint ones, the M820's are amazing but then again not everyone gets the chance to demo them right away.

If you think the SLX is good but still fades, try Zee or M810...M810 especially will not fade. I made it overdramatic but you know me, it's either 20000% try hard or nothing lol. Always a bit overboard on things.

The M810's are straight heatsinks and the original levers are more than good. Some ppl still prefer them over the Xtr style, it's all preference.

But I would say M810 (as they have crazy good deals on them). I doubt you'll have to upgrade but if so, Zee levers or M820. I would stick to DH levers more so. XTR does great, but knowing ZEE/Saint now, that would of been the slightly better choice. w/ non-ice 203mm front back rotors (Icetech is MUST have, reduction of heat is insanely unbelievable), I can drag the brakes all day on a very steep course and will not fade for a moment.

 thanks for the info! i had no idea that those ice tech rotors were so effective, and cheap! i will defo pick some up, ill also probably snatch up some zee hubs too. im looking at getting saint calipers with zee levers. i can get two levers and two calipers for the price of one saint brake.. im sold!

 I've done my own tests...no need to do fancy measurements with heat-infared sensors. It was SO obvious with M810 setup on both bikes, one with Hayes Rotors v8 vs. Icetech rt86 203mm. Icetech was way cooler to touch, also it took icetech about 2-4 seconds (resting/non-moving) to get to ambient temp. while Hayes took a good 10 seconds roughly. Like their stats said, about a 50% reduction which correlated with the average time to cool down. So yes, very very effective rotors + the heatsink capability of M810 = Way more than enough.

 The levers? Or grips? Levers are very nice. Super solid and smooth, but extremely powerful. Works nice paired up with m810 calipers. Grips are one of the best imo for ppl with bigger hands.

 I'm trying to sell these as I want same but in black w/ Red clamps instead. By any chance, would want these? The grips are in perfect condition and haven't worn down in the slightest.