IMHO, in order of Least Cost, assuming you DIY.
For Horse Power:
1. Lose some weight, simple but effective, lighter she is the faster she will go
2. If you have the OEM Air Box, cut open the Air Box cover, if you want to keep or go back OEM then find one at the salvage yard and cut it out. It will give you about 5 HPs, on the cheap.
3. Cat By-Pass, Used Burch is the least expensive, it will give you a bit more HP on the top end, but you may lose some low end.
4. Keep the valves properly adjusted every 12-15K miles, for heavy use like DE's a bit earlier, Properly adjust valve will maintain HPs
5. Steve Wong, Chip used if you can find it, if not save for one
From here slope gets very slippery.
Again this is my 2 cents
and what I would do if I had to start again.