I don't know about back pressure being good or not, but I think an x or h pipe run is suppose to be slightly better. I wouldn't think it's all that much and you should be fine to leave it as is. 2.5 is more than plenty large. You want the engine to BREATH as much as possible (e.g. exhaust pipe).
Here's something someone wrote up:
"This has the most effect when there's a bunch of time between pulses...in other words, at low rpm. As the revs rise, the pulsed flow becomes more and more like constant flow, and the scavenging effect is diminished. So, at low rpm I need a small pipe to maximize scavenging, and at high rpm I need a big pipe to minimize pressure drop. My exhaust pipe can only be one size, so it's a compromise. For a given engine, one pipe diameter will make the most overall power (i.e., have the largest area under the curve on a dyno chart)."
Read the rest:
http://www.dsmtuners.com/forums/freq...kpressure.html