Mike,
I have a stock MkI and was shooting bowling pins last summer, the hot weather caused my hands to sweat and the pistol grip was slippery. Tried to find some rubber grips for it but had a hard time, seems like the grips are mostly for the MkII's. I ended up slipping a piece of bicycle tire tube over the grip to get a little more traction and it worked for me (doesn't look real nice though).
I think the rubber grips for your MkII would give you a better grip and probably worth the money, thumb rest.....just your preference I guess. If you could find some way to try out a set of grips before you buy it would be a good thing, for shooting (as opposed to appearance) its all about the "feel".
Good luck,
Roudy