If someone already has a reasonably modern sportbike in good working order, he/she will almost always be better off to spend any surplus cash on a good riding school (Freddy Spencer, Keith Code, Reg or Jason Pridmore etc) rather than getting hosed on the trade-in for the latest and greatest. You will get a lot more preformance improvement for your $$ from a good riding school than from any other investment.
Of course, if you are already an expert (say, you routinely finish on the podium in your local club races) or if you can afford to spend the $$ to have the latest and greatest bike AND take the riding schools, go for it!
On either the street or the track, a good rider will almost always beat an average rider, regardless of the bike.