Go for the O/R X-pipe.
Here's my experience. I had FMS shorty headers with Bassani O/R x-pipe. Read alot about how good the Pro Chamber was. So when I upgraded to Mac LT's I ordered a MAC Pro Chamber. The fit was just a little off matching up to the LT's.
The Sound -
it rumbled, but not what I expected. I noticed a big jump in engine temp ~ 20 degrees or more.
Performance - Torque was up in the low range, and some what in the mid range. I had it dyno at 281.1rwhp / 308 lbs torque with no chip, and timing @ 10 degrees and the LT's.
I had the O/R X-pipe put back on and the engine temp dropped back down to where it originally had been.
Performace has better with the x-pipe. a little less in the low, but the mid to high screamed !!!
Sound - The o/r x-pipe wails at 3000 rpm's. People turn an look. I drive thru the parking of Publix's and the guys bringing in the carts stop me and tell how good the car sounds. With LT's it's not as raspy as it sounds with the shorty headers.
Looking at the ProChamber on the inside it's like a big hollow block wiith inlet and exit points, but it looks like the exhaust would just bounce around in there.
Just my 2 cents here, go for the o/r x-pipe, you wont regret it.
Dewman