Solved

Doctor what’s the point?

I had a 1star doctor now I have a 4 star doctor, I can’t see any improvement in health still at5% an hour just costing more money , or am I missing something?

For Doctors only full stars matter and they will improve your training by spending less health during training sessions :
5 star => - 10% health
4 star => - 11% (same for 4.5 star)
3 star => - 12% (same for 3.5 star)
2 star => - 13% (same for 2.5 star)
1 star => - 14% (same for 1.5 star)
0 star => - 14% (same for 0.5 star)

And drivers lose less health during races which is very important if you're racing in a competitive league or races are 100% length

Doctors don't reduce health lost in a race.
Sam's answer is correct.