I have a set of Marschal grips with the finger cuts on both panels and the thumb rest on the left panel (I am righty). If I hold it with my left hand the thumb rest does not interfere with shooting, nor is is uncomfortable. I can feel the thumb rest between the base of my index and middle finger (on my palm), but I can still grip the pistol just fine, and I have shot it left hand a few times without trouble.
However, if I had it to do over again, I don't think I would get the finger cuts, or maybe only get the cuts on the left grip so that the tips of my right fingers would rest in them. I had to adjust my grip slightly to get comfortable with the finger cuts on the right grip, and I'm not sure I like it.
That being said, I really like the looks of them, and they are high quality. You won't be disappointed by the fit and finish.