Bigger exhaust = LESS backpressure = LESS lag and MORE power sooner.

More backpressure will not decrease turbo life or motor longevity in this case.

Your 2.36" piping should be fine for your setup. I wouldnt worry that a 3" is going to make much of a difference. Nevermind with that Greddy kit your DP is like 2" anyway, so you still have that bottleneck.