Cross Class Penalty = -40%
Rank Penalty = ~-20% at 1st rank difference and about 2-4% drop per rank after. Varies from equipment to equipment because scaling formula is such that the breakeven point between 2 consecutive equipments in the same series is set at the halfway point of the optimal ranks.

E.g. You get more stats from a R12 tool than a R7 tool at R10.

And the penalties stack. So if you're both of the wrong class and rank, you can get 60+% penalty.
I do not know if there is any penalty cap.