maximum exhause sound and performence

i have a v6 stang that has the factory y pipe w/ dual exhaust. i took her down to the local muffler shop and we installed dual straight exhaust w/ no mufflers....it sounds really great in the low RPMs (however there is no ricey output coming from it in the high RPMs) but i've heard a guy that has the exact same setup except he has a H-pipe

I need to know what will installing an h-pipe do and how many of the 4 cats will it eliminate and will i see any horsepower gain.