For staff, only the star rating is relevant, as of today.
Regarding car research, it used CD and TD star count to provide the research power.
With the same 4.5 stars TD, using a 4 stars CD or a 5 stars CD will not provide as much power.
It also depends on other cars development
Referring to Joey McLane
guide:
Spying on 2 attributes with 20% research power :
- for 2 points difference : 2 x 0.20 = 0.40 => + 1 point
- for 9 points difference : 9 x 0.20 = 1.80 => + 2 points
Spying on 1 attribute with 40% research power :
- for 10 points difference : 10 x 0.40 = 4.00 => + 4 points (and not + 5 points)
- for 11 points difference : 11 x 0.40 = 4.40 => + 5 points
So quite a lot of parameters are influencing car development, so try to read some of the guides for more information.
I hope this helps.