Just put some slicks on your MTB and give it a try. I'm using a 1995 Trek 8000 fixed fork mtb with slicks on my 10 mile NYC commute and it is perfect for that use. You are unlikely to find anything better that doesn't cost a lot more money, and it isn't expensive to put slicks on. My slicks are WTB Slickosaurus 26x1.5 and they made commuting on this bike easy. You're not looking to win the TdeF on this. You'll be surprised by how much a difference slicks are over knobbies and you'll stop looking right there. Locking out the fork will help too, so maybe have someone look at that.