Auldyin you should be fine with your set up as described. You could have 10 divertors above your thermostatic control valve and you would not get any more water out of it. If you had a second thermostatic control valve for say a rain head and the other for a hand held and the tub filler you would then get to much water I think for the 1 1/2" drain line.
Most tub's here in Vancouver that we find in high rises are piped with 1 1/2" copper overflows and ptraps. Make sure it stays clean.
If you have the ability to upgrade the ptrap and overflow to a 2" line you will love the drain rates. My daughter's tub is plumbed like this and works great!