It still depends on too many other variables to give a general estimate of "this is what most pro racers are capable of". Your best bet is to look at time trial events of the pros and see what their times and average speeds are--then you will get a better idea of the speeds they are capable of holding.

The biggest difference between "us and them" is that they have the capability to produce massive amounts of power over far longer periods of time--hence the 6-8 hour classics races and tour stages. When you look at US Pro racing, you get slightly shorter events for the most part, but still longer than most of us can handle if we were even capable of the same power outputs in the first place (which many riders are).

Here is an illustration: watching the 2004 Tour de France DVD, there is a stage where a rider is away for almost the whole stage, and ends up winning by a matter of seconds over the chasing field. His average speed over the last several kilometers was well above the mid-20's, if not above 30mph. Now, this is a speed that some people on this forum are capable of holding on their own for a short period of time--but not after 4-5 hours of riding flat out by yourself.