I ride a converted hard tail 20 miles everyday for a commuter / town bike. Works great. Though I would not recommend for legit group road biking unless you need to handicap yourself because you're riding with a group below you. But a few cheap changes made a big difference.
Key mods for speed I made, over the course of time, in order of perceived impact:
- slicks (as stated in earlier posts) (~$40)
- road cluster (11-25 for me) (~$35)
- rigid fork ($55)
Only other speed mod would be clip-on bars, but on a pseudo MTB I think they'd boost the freak factor up to near recumbent level and I don't like hunching over that much given the frame angles. That's where I'd buy a new frame. But I do get another free 2mph whenever I drop out of the upright position.
Wouldn't ride without clipless so can't tell you where on the pecking order they stand.
Get a cluster with an 11 small gear, as you have will still have too much bottom and need all the top gear you can get with just a 44T chainring. As is I can go to around 29mph before I'm just spinning too fast, so haven't felt the need to mess with the cranks and chainrings.
The fork was from Nashbar, about $55 for the part only, probably another $40-$60 if you don't do your own work. Bigger difference than I expected, don't believe the crack head who says suspension doesn't hurt you. Key measurement, other than the 1 1/8 or 1 steerer, is the axle-to-crown height so you don't screw up your frame geometry. Get that right, and just buy the cheapest one you can find. There's plenty of give in the MTB wheels and your arms for road riding.
Other mods for commuter comfort / safety, so maybe you don't care:
- fenders!!!!
- rack / panniers
- lights
- reflector stickers all over the damn thing
One thing left I covet -- since I already have disc-ready hubs and the new rigid fork has IS mounts, I may spring for a front Avid BB7 for those wet days. No mounts on the rear, so impractical to lust for disc all around.