Find video clips of the World Challenge Cadillac CTS-V and compare them to clips of the World Challenge C5 Corvette. One is not running an X-pipe but both are running the same engine and dual exhaust pipes.
As I understand it, an X-pipe is designed to improve mid-range power. An H-pipe is a cheap version of an X-pipe. A Y-pipe just reduces the cost because less overall piping is used.
On the other hand, I'm not convinced that a properly built single exit exhaust with Y-pipe is any worse than a good quality dual exhaust setup. The problem you run in with a Y-pipe is the need to run very large piping for high horsepower applications which can cause fitment issues.