Shifting is just about acceptable on 8 speed with XT thumbies but not perfect. Every time I've tried it, it ends up 7 speed again quite quickly, it's just enough out to annoy me! YMMV
I fully agree with LGF, get some Suntour XC Pro/Expert Accushift Plus 8 speed thumbies, they are fully Shimano compatible (some 94 Konas used this combo), a bit smaller/neater looking than XT/Deore and work perfectly!