well there's two different things going on here. You have road tire slicks and mountain bike tire slicks. IF you get the road slicks it will require you to have different sized rims and depend on what kind of break set up you have they probably wont work but that will add extra MPH to you overall total.
Mtb slicks on an mtb rim will also add extra MPH to your overall but not as much as the road/road set up.
I did an MS 150 (houston to austin 182 miles) two years ago on a mtb with slicks/mtb rim and it was coush.
So in summation:
Yes it will add extra MPH to your totall but how much will depend on your setup.
t