I do all my SIT training seated at high cadence, 120-135RPM.
I saw general wattage gains across the board, mostly in the shorter durations under 3min, but even FTP did get bumped up a few %.
More importantly, I noticed my IF/NP going up substantially on longer rides, e.g. 0.80+ over 5 hours, due to drastically reduced fatigue from hard efforts. I’d hold Z2 and tempo for most of the ride, but even 4 hours in, with adequate fueling and hydration, I could still put out very strong short efforts, significantly bumping up NP.