If you're thinking racing anyway you've got to have deeper (on average) pockets than someone just building for fun flying... So i'm curious: Why not a Rocket?
There are many SARL racers that are flying mostly stock airplanes. After guys get involved and get hooked then the bug sets in and modifications start. A lot of those modifications cost next to nothing other then time. By that I mean faring, cowling, wing tip mods etc.
Engine changes are never inexpensive but there are many non racers that have higher compression pistons etc.
As I stated in an earlier post much of the challenge is becoming a better pilot. I would take a look at my "track" after some of my early races and I was all over the place. We have all gotten use to flying the "magenta" gps line and there is a lot of lee way for side to side errors when you are hand flying a race course. These little errors add up to extra distance. Auto pilots are not much use on some of the courses that have a lot of turns and thus you have to do a better job with heading! just like we used to do before gps.
Turns provide an area for constant improvement. Not every plane, or pilot, handles turns exactly the same way. Do you go wide, not to burn off speed but use more distance, or do you turn tight, with g's, loose speed, but cover a shorter distance? These few simple things can, and do, add more to your speed then $ thrown at the airplane.
Many races have been won by simply guessing better at upper winds; is it worth the climb for another 2 knots? stuff like that.