In short, I used to do threshold work and above all at TT FTP but have found better results having 'true' power zones and accepting that when doing TT position work I may have some sessions where I can't hold it for the full interval. I don't see any benefit in high intensity work done in position at 90% of what I can do, as I am so rarely racing above 110% FTP in position other than accelerating out of turns etc.
In short, I used to do threshold work and above all at TT FTP but have found better results having 'true' power zones and accepting that when doing TT position work I may have some sessions where I can't hold it for the full interval. I don't see any benefit in high intensity work done in position at 90% of what I can do, as I am so rarely racing above 110% FTP in position other than accelerating out of turns etc.