I just installed a Vorsprung Corset on a rebuilt stock RP23 on 2010 Enduro and it helps noticeably. Not as good as my stock Monarch/Debonair on my Nomad3, but still playing with set up and tokens.
So does a rockshox monarch go straight on after you adjust the stroke length ?
you have an x-fusion coil on it now I assume? I don't think you can mount a monarch plus directly. The evo link you have is designed to be run only with that shock or a proprietary fox dhx rc2... if you google there are some forums here and on mtbr of guys doing mods to make a monarch plus, or other shocks work.
edit: i just wrote the above, but I'm less sure about it now... I know there is an evo kit that is designed to run with the fox dhx, but I don't know about the x fusion...