i don't think it goes any faster.. well not by much anyway, but it sure is more efficient reduce sound energy, much quieter and of course more grip
If by not going any faster you're saying the top speed won't be any higher, you're probably about right. What I did notice when I put slicks on my hardtail for commuting for a while was that I ended up with a higher average speed (usually by about 3-4 mph) over the knobbies, since pedaling effort at any given speed was typically lower. I used Nimbus Armadillo 26x1.5s @ 100psi. It did kinda suck to have slicks on the bike, though, since I ended up having to switch the tires every Friday night for the weekend mountain biking.