SunTour thumbshifters work pretty well with both 7- and 8-speed Shimano-spaced cassettes. SunTour cassettes are spaced at 4.8mm for half the block and 5.0mm for the other half; Shimano 7-speed is 5.0mm and 8-speed is 4.8mm for the whole cassette. So, get things spot-on in the middle of a Shimano cassette and they'll still be close enough to shift okay at the ends. Remember that Kona used to spec bikes with 8-speed SunTour thumbshifters and 8-speed Shimano cassettes and no-one seemed to complain.
I'm currently running SunTour XC-Pro 8-speed thumbshifters with a late-90s XT rear mech and a SRAM 8-speed cassette and the whole lash-up works a lot better than I ever expected it would... at least when it's clean