According to www.cycle2max.com, you have to be shiddot to sustain over 300W. They have a 300W club there. My best (according the that ref) is about 260W averaged over about 20min. Then I'm blowing like a whale and pouring sweat. And I regard myself as pretty fit at 45, not master level, but fit to very fit. A mate of mine is just short of 300W at 298W, and it took him years of constant training to get there.