The DG pipe actually takes away from some of your power. An FMF exhaust would be a nice step up from that exhaust for about $150-$200 used, but if you want to be racing then a Toomey or F7 pipe is the best way to go for around $375. Another good idea is to get a high flow UNI air filter to go with that, take your air lid off and rejet.
Then send your head and cylinder off to a place like Flotek, Ken Oconnor racing, SCD racing to have your cylinder ported and polished, and have your head machined. The head machining along will increase your HP about 6-8. That's the big ones, after that you can gain little by little by doing other mods like, lightened flywheel, timing advace, etc. Hope that helps!