Pete
I believe overtrained drivers get ballast weight added so that'll be why it seems like their performance is dropping off. I'm sure I read it in the guidelines/rules.
Ballast doesn't depend on driver's level, but on manager's level. It gets added when a manager above level 8 participates in a beginner tier and above level 14 in a pro tier.
1kg per level above the max + 1kg is added.
Ex: a level 18 manager in a beginner tier would have 11kg ballast added (10×1 + 1), in a pro tier he would get 5kg (4×1 + 1).