1st question! - can a generic K & N style filter (i got mine from procarparts) actually restrict/strangle airflow to a GS-T engine in compared to the air box? This filter is made TO FIT the gs-t, no adapter needed, but it is not as long or large as a couple of the other ones i have seen.

Second question! - if i am going to order a muffler off of ebay (not a whole exhaust system), then do i have to order a muffler that is labeled as a GS-T one? (stock is REALLY quiet and lame, i want a real sound) Can i order an "eclipse muffler exhaust" and still have it have enough airflow??

#1 yes is possible but not likely unless its a really crappy filter.

#2 You can get any muffler you want as long as it fits under the car, there are many universal mufflers to choose from, just have to get it welded on.