I've just gone through this exercise whilst trying to sort myself out the best bike option for my first triathlon. To start with I went from MTB knoblie tyres to MTB slick tyres which made on average about a 2-3mph difference. I then managed to get my hands on a mid range road bike which again increased speed by a further 2-3mph over the MTB with slicks, so all up over a MTB your looking at roughly 4-6 mph speed advantage. Obviously there are factors that effect the speed gain e.g. weight of the road bike etc but from a mid range MTB to a mid range road bike that is the difference I found. I've just moved up to a higher spec road bike that is much lighter and once the weather clears up am looking forward to seeing if there is another speed gain to be had.
