Dual cats, or mufflers for that matter, do not promote good power. The gasses mingle in them causing flow degradation and extra turbulence.
Stay clear of them.
The Y-pipe is used so only 1 pipe has to "snake" it's way to the rear of the vehicle. It is only then split back up into 2 pipes (SVT) for looks. A properly designed & built single exhaust (3") would produce the same power gains.
If you do go true duals. Make sure not to have the pipe crush bent. I've seen quite a few terrible looking dual exhausts on this site!
You would get better performance out of a properly built single or even quasi vs any poorly built (crush bent) dual.
