I highly, highly doubt you'd notice a 10mm difference between the bars you have now and the ones you want. If I were you I'd jump straight up to 800 with lock-ons and just experiment.
As my experience of riding has gone up my bars have been getting slightly wider. I've also noticed that with steep XC bike geometries I prefer a narrower bar -- like 650-680mm. My slacker AM bike I now use a 725mm and could even go a little wider I think, maybe 750mm next time. Stem is usually around 60-70mm. For reference, I am 5'10" with wide shoulders for my height.
685 on a 150mm ride would have liked to go a little wider buts its carbon, i got it for a ridiculous price, and that size has always been comfortable for me i'll keep an eye out for an upgrade but an equivalent 720 bar would break the bank