I've bumped into a realistic ceiling of about 220w twice now. In 2010 I had a massive year and based on race and test numbers 220w was a realistic FTP. In 2015 I had a decent year and my best test was 218w.
I bet if I trained better and didn't work etc I could be better. However I am pretty sure that my max wouldn't be, say, 275w. Trying to do tests there's just no way to sustain the numbers required to test out at 275w. It's close to 30% higher than my best FTP.
Ceiling numbers, unless the coach knows some secret that no one else knows, is an educated guess at best. No one has been able to publicly quantify max FTP for a given rider.
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"...during the Lance years, being fit became the No. 1 thing. Totally the only thing. It’s a big part of what we do, but fitness is not the only thing. There’s skills, there’s tactics … there’s all kinds of stuff..." Tim Johnson