Gears are definitely #1 (although I STILL havent changed mine out...lol).
On the plenum, I would go buy a dremel and hog out your stock one. I've done 3 or 4 for folks (including myself) and each has made a noticeable difference.
__________________
Dave
1998 GT cp (totalled, compliments of a dump truck)
1996 pony (awaiting transplant)