Other than factory, I think the staff numbers only affect the salary costs. I have not tried if the staff number in design center directly speeds up the part development or something, though?
I'll continue my GT game to see how tough it is with the marketability. So far after 2 seasons I'm still at 1 star sponsors, with 1.5 star team rating I think. Then again, the marketability of my pilots sucks atm.
Random weirdness to report: My pilot was out for 24 weeks (I think) with an injury, and used the reserve pilot during this time. When I dropped the pilot to reserves, he had 2.5 star overall (not taking the injury into account), and when the sick leave was done, he had jumped to his full potential at 4 stars! To make things even weirder, I happened to renew his contract while he was injured, and he was happy to accept ridiculously low salary, probably because of the reduction in his skills due to the injury... now I have a 4 star pilot with under 120.000€ salary, and he also happens to be pay driver, so actually he is paying me ~130.000€ per race.