I agree with aaronmcd.
You could put a bunch of money into your Haro MTB (new fork, new bars, tires, etc) and it will still be an ungainly frankenbike. Whereas, it's a decent MTB right now. Keep it for hill training! And fun!
You should be able to find a decent, used road bike for $200-300. An 80's or 90's bike with new tires, cables, and brake pads (full tune-up) would ride way faster than your Haro, even after modifications. If it needs a tune up, lower the price significantly.