Again, last night, I saw evidence my theory of 3/3+ stalls the game proven correct. As soon as I asked and we went to 1/1, the cart started moving again, imagine that. /facetiousness.
There's already a cap of 3 spies but I don't think such a cap exists on snipers, I know for a fact I've had class full messages on spy. Hell once I was the only spy then all of a sudden I was forced to pick another class in between deaths because all of a sudden there were 3 other spies.
Forced class balance is a very slippery slope, if you cut down to 2/2 you could end up with two shitty randoms and some frustrated main who can't play his desired class. I think a restriction to 3 snipers is needed, or even better a max of 5 between the two classes though I'm not sure if that's possible.
The other thing is spy & sniper heavy on Red can work especially when defending a final push point, but on Blu it's absolutely detrimental to pushing the cart when Red has any sort of organized defense.