[Lonnie P]

Actually, the rule of thumb & with many charts published by exhaust & muffler manufacturers state 450HP is about the limit for dual 2-1/2" exhaust.

Also, you want to retain the 3" off the headers as it will act as a collector extension, boosting your mid-range torque... especially since your header primary tubes are somewhat short.

If anything, I would stick with what you have, but possibly go up to a single 3.5" tailpipe after the mufflers.

 

[Lonnie P]

Given the option, an H-pipe will mellow out the sound somewhat with the potential for increased mid-range torque.
The X-pipe may offer slightly better torque, but it will have more of a higher pitched/raspy tone than the H-pipe.

Keeping the mufflers towards the middle of the vehicle (usually just before the rear axle) tends to work best for sound control & always at least 2ft from the header collectors.