Although there is only a ball hair's difference in cassette width between the two brands, the Suntour thumbies actually pull less cable per click and the Suntour rear mechs require slightly less cable pull to move between each sprocket. This is where you'd be more likely to encounter compatibility issues. So, your Suntour shifters may not move your Shimano rear mech quite enough to cover the distance between the sprockets of your 8 speed cassette. That's my experience anyway.
There are ways around this though, such as using the narrower spacers from a 9 speed cassette in place of 8 speed spacers. I've used Suntour XC Expert 8 speed thumbies with Shimano M735 and M751 mechs on a 9 speed cassette with great success. A cheeky wee over-shift past the last click brings the thumbie onto its friction plate, which I found was enough to hold the derailleur against its limit stop over the biggest sprocket.