I don't think your gearing is too low for you unless you constantly find yourself spinning at 100rpm on the small cog large chain ring combination. Chances are you are just pedaling too slow.
Thinner tires will be a difference you can feel, especially with smooth tread. If your wheels take tires 25cm and up in width then you might want to look into changing rims to something that will take 20-23mm, but that's up to you.
No clue about computers since I still don't have one. But one of these days I'll get a garmin when I feel like spending 300$
Changing the fork would make you feel the road more and might increase efficiency since a lot less of your weight transfer between pedal strokes will be going into the shock. I have a Carbon fiber fork that I am trying to get rid of, PM me for details if you are interested.
Overall you'd be looking at probably around 300$ spent if you spend 200$ on wheels and the aforementioned fork and a computer. Cassettes aren't too expensive either, and I have a triple crankset (no bottom bracket) laying around somewhere. I actually have pretty much a second bike in spare parts, so if you want to know more send a PM.