Except it's possible to muffle a high performance setup without major losses. Done wrong, a muffled setup will lose you big, done right it should have little to zero impact.
Doing it right is tough though without having all the data on-hand necessary to engineer it properly and there's no one-size-fits-all approach because the right system for a 350hp 318 is different than the right system for a fire breathing 408, and using one on the other will likely have drawbacks in both cases.
A cross over only makes sense on a car that's getting a full-length system already - which rules out a track-only car. If it has a full length system and you want it turned up to 11 (as far as volume) then don't use a cross over. If you don't care how loud or want quiet, and/or want to maximize low-end, then a cross over makes sense in most cases.
