Can't say about Alum vs. SS, but TTI tech said when I bought my system that 2.5" was plenty enough for 500HP or less. The 3" system I have looks bad-ass under there, and even though it fits really well, it does occupy a lot of space, and clearances are minimal. Springs, tank, shocks, and brake hose are factory located ('72 RR). In retrospect, 2.5" would probably have been better.

BTW, wife's '09 Toyota has factory mandrel bent exhaust, so there must be something to it.