From my experience Level 8 for Rookie and Level 12 at are the top that can be someone when playing normally from the start before moving up to higher Tier.
So according to a) Max Design Level and b) Max Driver Level
at Rookie and Pro a) Design should limit to 40 per Attribute at Rookie and 60 at Pro
b) As for the Drivers If possible his stats to count the same regarding his level. i mean his attributes limit to 8 for Rookie and 12 at Pro no matter if he has 15.
I don't know if is possible to set this for drivers with out stopping his development.
Thanks in advance