very good point and ive thought of this myself.
your basically asking, is the increase linear?
I can only make the assumption that it is. Ratings appear to be linear, and in general, pops in a stat category appear to make a linear difference to a player. This would support sub-levels, etc etc..., BUT, the argument against this, would be that salary increase is not linear.
So perhaps its not? I dont believe anyone actually knows the answer to this.
I thougt how this would affect any PL where you have scrubs testing SB out - and how you could apply any general concepts across the board.
Say in the game where the SB on the players goes from Inept to Medicore, makes a MASSIVE difference. But maybe its a tipping point? Maybe once your defensive players hit a certain point, its just gets better and better exponentially.
SO yer, it could very much affect this. But i can only work on the assumption that its linear.