OK, I think its time to get a bit into this a bit more.
Anytime you open the exhaust using after-market pieces, you will gain hp, but you will lose some torque. It all has to do with the loss of back-pressure in the system.
A near stock engine would not run very well with a 3 in. diameter pipe from the h-pipe back. You're just not putting out enough power, and the lack of back-pressure would take away some torque numbers.
The 5.0 started to shine after the install of a new H-pipe and cat-back, due to the design of the push-rod engine and the exhaust system.
The 4.6 is a different beast. Its an engine that loves the high revs, and can really pull in the upper tach with a few mods.
When you pull that restrictive h-pipe with 6 cats off, you WILL notice a small decrease in torque on the low end, but you will also notice a new chunk of power up high. That's why the 3.73 gears are so important, they will give you a "bite" that you've been missing, as you'll be able to use more of your power band, and you'll be in it more often. An h-pipe will retain more of that torque after those restrictive cats go away, as an x-pipe will give you more up top. Like I said, its a matter of taste.
The 4.6 is not your uncle's old 5.0, and you can't drive it like one either. Take the car and use it where it wants to be, high on the tach...
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"The Yellowjacket"
1998 Mustang GT
13.97@103
"Traction Impaired" or "Nitto Needy"
"They think they can make fuel from horse manure.... Now, I don't know if your car will be able to get 30 miles to the gallon, but it's sure gonna put a stop to siphoning."
- Billie Holliday