Comparing average speeds is like comparing apples & oranges. I find average speed is usefull for tracking long term progress on a given part of a given course - for example I track average speed on my 6AM morning ride after I have done 1 mile of warmup and then after I crest the second big hill 15 mile later, then follows 2 miles of cool down which at the end I don't care what my average is. On this ride my best average has been 17.7mph and that was last year. Hermes is right - average in general is not a good guage of what your ability as a racer will be - maybe in time trials only. However average is a good long term indictor of progress if you are taking the average of the right thing - same course and your own individual effort.
You ask is there some barrier at 17MPH, my answer to that is most likely yes. I suspect the barrier is again the law of averages. Most of the B group riders I know are 16 to 17 mph riders. It seems that this is what the normal fit person can achieve given a normal to moderate training regime and some time working at it. The faster riders I know are like Hermes, they train harder, put more time into it, race and are in general more serious about speed. They can achieve more. So I would say, if you want to go faster you will need to train to go faster and you will have to continue to train to sustain that.