And will cat-back or "cold air" make a difference?
Absolutely not, especially the "cold air".
That's what I told him. That he probably would not notice the difference.
Well, you tried and that's the best you can do. If he/she wants to dump $1500 for nothing, oh well.
Really, all that's left on a modern vehicle to make a noticable difference is to cut the catalytic converter out. It worked for me, but wow, is it tough to outsmart the downstream O2.
Otherwise, electric fans, efficient transmissions, great cylinder heads, precise timing, injection, cold air from inside the fender, it's all been done for you. Read that in Hot Rod in the mid 90's and they were sure right.
