I'm running 9 speed cranksets with 8 speed chains, cassettes and shifters on
two MTBs right now - an XT FC-M760 w/ the integrated BB on my hardtail and
an XT FC-M752 splined BB set-up on my full rigid MTB. I've noticed no real
difference in shifting performance compared to full 8 speed setups. I'm
also using a 9 speed front derailleur on the hardtail with no fuss.

The only real problem I have is with my splined XT crank/BB setup. It
developed an intermittent clicking noise soon after I installed it last year
that I just can't get rid of no matter what I do. All the trouble-shooting
I've done (including wapping parts around from bike to bike and replacing
the BB) has points directly at the splined BB/crank interface. No amount
of cleaning, tightening or greasing gets rid of it permanently.