Will a racing bike make much difference?

For a given effort, the difference in speed should be quite noticeable. But there's more... A road bike offers quicker response on pavement than a mountain bike. When you accelerate from a stop, you should find yourself quickly at speed. The short wheelbase and steeper head tube angle of the road bike also allows quicker turning. True racing geometry bikes can often feel twitchy or "nervous." The lower riding position also brings into play more muscle groups than you use when riding in a more upright position. Not only are you more aerodynamic, but more muscles are digging in to maintain a better speed with less effort. Don't discount the Allez as a bottom of the line road bike. The geometry is almost identical to the Tarmac, and the ride and performance are very good for the money. If it fits, it's a very good choice.

Others have addressed the differences in speed. Another significant difference is that road bikes are more conducive to long rides. You can avoid hand palsy by using the many different hand positions available. Just sliding a bit back or forward on the seat makes a big difference. I have no problem with 100 mile rides on my road bike. I wouldn't want to do them on my off-road bike. HTH

I owned both a mountain bike and a racer as a boy. There is a big difference. racers are much faster on tarmaced, flat roads and things, and it takes alot less effort to go fast. they also, usually, have simpler gears which are less hassle to use. mountain bikes are better off road, on dirt tracks and things racers are practically useless. also things like potholes, stones and kerbs which you'd skip over without thinking on a mountainbike are of alot more concern on a racer. If you live in a city, or are using the bike on roads, you're better with a racer i think.

I have a mountain bike based hybrid, with road tyres and gearing, for day to day use, and a road bike for the summer. Whenever I ride a similar route on the different bikes, I consistently get faster average speeds on the road bike by about 2 to 3 MPH, even though my maximum speeds down hill are similar. I use a GPS unit to measure the speed and distance, so it's fairly accurate. The tyres make a big difference, as does the gearing, but everything else, such as weight, and lower wind resistance, since you are lower on the road bike, all contribute. I work on the basis that if one bike is harder work, then it's more of a workout than the faster bike.

no

The #1 problem with racing a mtn bike vs. a road bike is that you can get into a more aerodynamic position on the road bike. The gearing is normally not such a big deal depending on what type of racing and on what type of course. Another issue is susp. movement, if you have lockout use it, any unwanted movement wastes energy. Don't worry about bike weight unless you are doing some long hard climbs involved.

A road bike (what you call a racing bike) will be faster than a mountain bike on pavement (although slower on trails), but the degree of difference will depend on many things. Road bikes are faster partly because they are typically lighter (NOT a major effect on speed on flat ground, but can matter on hills), but also because the rider position can be more aerodynamic (major effect), and the thinner, lower tread, higher pressure tires provide lower rolling resistance (major effect). That said, on relatively flat ground, a light weight mountain bikes with narrow (e.g. 1.75 in), high pressure (e.g. 70 psi) slicks can be almost as fast as a low end road bike. So, you are likely to see some speed increase on paved roads if you switch, but you will also feel quite a different ride. For example, the road bike will be more responsive (and twitchier) and have a harsher ride. I have both good road and mountain bikes and can say from experience that on smooth roads with the same effort level I only go 5-10% faster on my road bike. It's a common misperception that mountain bikes are much slower. That's only true if you have knobby tires. Hope that helps.

Huge difference. Most (but not all) is due to wheelsize differences, they do make a 700cc wheel that will fit most mtn bikes, it's the Mavic Speedcity wheels.