it is a common misconception that exhaust needs some back pressure. the problem with using an exhaust that has too large a diameter is that it slows exhaust gas velocity. part of the header's purpose is to balance exhaust gas flow and actually use one cylinder after another to create a vacuum effect, pulling the gas out as the piston pushes it out. if you wanted back pressure then you could just stick something in the way and keep the large diameter piping, it would crap out the whole deal.

2.25 is fine for your avg exhaust, but i would also recommend a mandrel bent cat back setup, i loved my hks super dragger, it was 2.25 with a cannister muffler, and had a small resonator in the mid pipe (after cat, pre axle).

the 2.5 is also fine, but if you don't need high end breathing then you would just lose power on the bottom end due to the larger diameter on the collector, and if running it into a 2.25 exhaust its just a waste. stay away from this size for an LS

if you are budget minded than you could always just pick up a glass pack, magnaflow makes and excellent 2.5 diameter, in 18 and 24 inch lengths. get a mandrel 45 or 90 to make a turn down. for some reason the magnaflows flow better than a straight pipe.