yeah, im not picking sides here.... but the threory is that the equalization in pressure makes both sides flow more efficiently. The flip side best I cant tell is that its a straight shot right out the tip. I think with new science in the motor, the old ways of thinking have been rethought and different conclusions made. Magnaflow and such companies who r&d the crap out of stuff all run some sort of cross over piping. I will be running a X pipe, but I had it laying around anyway from a previous build.
All that being said, the headers are the weakest link if you do the rest and not the headers. 2" pipe into 2.25 (i think thats what you said) will sound deeper from the expanded size, but wont net you as much of a gain. You will see and hear a difference, but it wont match the same as doing a FULL exhaust. Most of the headers I saw were all a 2.5" collector, so think about that when sizing your piping. If you have a 2.5" collector flowing into a 2.25" pipe, it will bottle neck and reduce the exhaust velocity.
thats my .02, but like I said i dont care to pick sides over who is right and wrong. Building custom rides is an art not a science. There is no wrong.