NA you lose some bottom end torque going with 3 inch with no cats and header. No significant power gain in hp. Proven already. Other than that louder noise levels. Most go with 3 inch as a temp till they get a turbo. So NA only 2.5 would be the best over all.Sorry to be bringing this up from the dead…………
2.5” single exhaust supports +/- 230hp, if stock the IS300 2JZ puts out 218hp, then the 3” should be the go to as a base exhaust. No?
A “good” cold air intake, with headers (no cats, even better!) and a 3” exhaust with a high flow straight through muffler (like Borla?) should be able to more than exceed 230 +/- hp. Why would anyone want to bottleneck the output by going 2.5”