I've been riding about 6 months now. All road work on a Cannondale M400 mtn bike with smooth tires. I'm up to 15-20 miles at a stretch now with an average speed of 14-15 mph.
Spent the other day on a long line with a buddy trying to get an iPhone(no luck) and the dood ahead of us was all 'geared' up. Spandex galore. Rode in on a Trek 1600. Sweet bike. Young guy who seemed in great shape and was all talk about quality bikes and proper riding outfits after I told him what I rode and wore(jeans/t-shirt)
He said I would get a 4+ mph avg increase just by getting a decent road bike and outfit. Went on and on about bike weight and wind resistance.
I agree with the change over from an mtb to a road bike. I don't necessarily agree with the choice of riding apparel. I ride as fast in regular shorts and a t-shirt as I do in my spandex. Any increase in speed due to less wind resistance from my clothing is going to be very small and doesn't matter much to a recreational rider. It would make a difference to a racer though. The only reason that I have any spandex at all is for riding comfort.