There is a lot of debate about the X-pipe vs H-pipe... However, one thing that is virtually universally known is the sound difference. With the X-pipe, the exhaust pulses are forced to blend... with the H-pipe, they stay separate. Obviously the X-pipe does a better job equalizing the pressure than the H. Generally, IF the H-pipe performs better than the X, it would be in your low end; offering at best, a bit more TQ down low. But a lot of times the low end numbers are equal... and when you start hitting the high RPM's, it's almost a given that the X-pipe will out perform the H-pipe. BUT... as I said, all of that is debatable.
For sound... the H-pipe will give a more low-throaty sound; more like an old muscle car. The X-pipe is usually a touch more tinny... or raspy. As far as I'm concerned, both sound nice but I do prefer the throaty-ness of the H. I went with an H-pipe on my F-150 when I had my custom true duals made... but, this truck is used for off roading and towing my boat mostly; I could careless if I lose 3-4HP on the top end; that's not where the truck runs usually anyway. I wanted the sound... and the small chance I'd have better low end numbers on the H-pipe.
Flow 40's?!?!?! You really like it loud huh? I have a friend running a set-up pretty much exactly like you're describing on his 95 Mustang GT; no cats, X-pipe and original flow 40's. The resonance inside the car on the highway with the top up is quite unbearable. To the point that he wears earplugs. Of course, his mufflers are just about directly under the rear sear floor pan and yours would be at the rear. So that will help a touch. But, if you must go with flow 40's... do yourself a favor and get the 40 Delta Flow; exterior volume is very very close to original 40 series, but they worked pretty hard to decrease the resonance in the vehicle. My truck has 50D's on it, and it sounds great... Depending how loud you want it, I'd say go with the 50's; I can tell you this much... you won't need to wear earplugs on the highway!
