The size of the pipe will give you very different results. On smaller displacement engines, having smaller tubes will help with low rpm torque (which small engines tend to lack) due to the the speed at which the exhaust will travel. Give that same small tubing at high rpm and you will create a bottleneck as the exhaust cannot evacuate fast enough.
If all else stock, just the exhaust piping diameter changing:
2" (stock) good low end torque but alright upper end power
2.25" (Magnaflow etc) low end torque drops ever so slightly but gain much more in mid and upper rpm power range
2.5" (other) low end torque hurts a bit but expect even better upper rpm power.
From what I've read, larger diameter exhaust piping will move your power band up.
Read up on Exhaust Gas Velocity and disregard the what people babble on about engines needing backpressure.
In all cases you will net a bit more HP and TQ, just don't think having a 3" on this N/A engine will give you the best high rpm power lol
2012 SGM Ti DCT Stock and loving it
OEM multispoke 17" with stock rubber summer-X-ice 3's on stuck rims = awesomesauce