I'd get a cross bike or something similar. Road hybrid, tourer, etc.

Barring that, I run 26x1.25 slicks on my MTB which I use for foul weather commuting. My commute sounds similar 9-10 miles, with more hills on the return leg. You could flip the stem, reduce the number of spacers or even change the angle of the handlebar for a more aero position but that will only help you go faster into a headwind or when you're already going fast. I much prefer having barends but mostly for comfort. Having a smaller gear (11t) in the back will give you a higher top speed although changing the large chainring will do more than changing the cassette.

I wouldn't bother reducing the weight of the bike - it's a MTB so it'll be expensive and not that valuable.