I thoroughly ported my stock ex manifold years ago before they had all the inexpensive ebay manifolds and so many used manifolds. I could feel a little difference with my SP61GT; Felt like it pulled a bit harder in the high RPM and lost a little spool in the low end. With that said, to me it is not worth the hours and hours I spent porting and polishing. Just get a tubular. But remember, you're going to lose some spool-up by switching to a tubular manifold while keeping the same size turbo.