There is another LS post asking the same question .... Having done some 4 and 6 cyl race motors , here are my thoughts ,for what they are worth ... Magnaflow sells a 2.5" ,cat back ,system for your LS . "X" pipe crossover ,2 resonators and 2 mufflers. They have posted dyno results 21 HP gain 20 FT Lbs torque increase..My thought is that 2.25" pipe would not change the results much , if any..most muffler shops can only bend up to 2.25" pipe so the big companies boast 2.5 and 3" systems , so there is no round the corner price comparison and compitition....