I do agree with the fact that allowing only 10 cavalrymen on the field in a 160-men battle is a bad idea. I really don't see 15 cavalrymen being particularly OP against line infantry. We've always done 15 cav max during pretty much every decent event, and this since Mount & Musket, and many times have I seen lines repelling the cavalry with a good formation. It all depends on which regiment charges which regiment, but 15 men is not OP.
Just my two cents, I don't mean to start any harsh debate. But I don't think you would spoil the infantry regiments' fun by allowing 5 more cavalrymen to partake to the event, while you would let the cav regiments enjoy it more.