Coggan originally stated, over a decade ago, that sweet spot “is a concept, not a zone.”
“Sweet spot” as a concept is about maximizing TSS within a manageable fatigue load. So, that depends on available training time – if you go back to the (in)famous chart, “sweet spot” could be anywhere from about 73% of FTP to 94% – it all depends on the size of the training dose.
So, the question is what kind of adaptations are you looking for, at what time in the season, and for what reason?
Long intervals, or long sustained “chunks” of 90-94% might be good if you want to slowly raise FTP from below, and develop more muscular endurance/stamina.
A lot of riding at 76-85% might just build up fatigue and not yield significantly better aerobic adaptations than just riding in upper zone 2.
I like the 90-94% stuff once or twice a week during base phases, then saving the work right at threshold for a the weeks prior to an event that prioritizes FTP (a mountain finish, or a TT). I’ve actually shifted a bit this year to cutting out zone 3/lower zone 4 work, since Base was over at the end of January. The hard days are either at or just under FTP (stamina-focus cycle) or zone 5 (VO2 focus cycle). Everything else, zone 2.
I’d like @brendanhousler to chime in here, but the spot in the week where I see a place for 80-90% FTP is after the hard parts of a quality session are done. Say, 45-60min of 96%+ FTP, then 5 minutes steady zone 2, then an hour of tempo. Or VO2 work followed by the tempo. If you’re limited to, say, a 2hr ride, then I think that tempo follow-up to the hard work helps prepare your body to ride in the break after the selection has been made. One of these days a week is enough, because it will tear you up pretty well – especially for a 40+. But, I think it does help you go deeper, for longer, in a 2-2.5 hour road race.
That may all just be “I think,” though.