Thumbhole stocks really don't provide any benefit. If the stock doesn't fit you, you're not going to shoot the weapon well anyway. The grip won't matter.
Imagine picking up a random golf club and making precise shots with it. A rifle isn't much different. Stocks are made to fit an average adult male. If you're 5'3" or 6'7", you're going to have issues. If you're talking about shooting to the accuracy of a marksman weapon, it isn't going to happen without proper fit.