I went from open exhaust with collector extensions to a TTI X-pipe ending with Dynomax UltraFlo mufflers just in front of the rear axle and the car went faster by between .05 and .10. Headers are TTI 1 5/8"-1 3/4" step on a 360. May require minor rejetting.
It added an extra 60+ lbs to the car, but I removed the weight from other locations to bring it back down to what it was. You shouldn't have to worry about it since you plan on a full exhaust anyway. Room to swap out center sections is limited, but worth it. I plan on using their bullet style mufflers next time to save room and weight, although it may be a bit louder.
To me, X-pipe looks more effective than an H-pipe because not only does it help equalize the pressure between the two sides, but the flow from one side can help scavenge from the opposite side at the X as well.
