Seiler’s 4x8 approach is not intended as threshold progression training. It’s better discussed as a method to raise VO2max, IMO, because it’s not a prescription of 4x8 at 105% on 2 min rest. It’s 4x8 maximal effort on two minutes rest, intending to spend as much time as possible above 90% HRmax - there is no prescribed power target. Essentially, it belongs in the discussion with hard-start/high-cadence VO2max intervals and the Kolie Moore protocol that we’ve talked about at length in other places.
I’m sure you can see improvements to FTP from doing a block of Seiler’s 4x8 intervals, just like I and others have seen improvements in FTP from a block of VO2max intervals from other prescriptions. Properly executed, they raise the ceiling, but I think that discussion is separate from threshold training.
My $0.02 is that I don’t see much practical utility in doing a bunch of work at 105%. Over-unders have tremendous utility for race prep and there a few different ways to achieve that lactate shuttle, and those should make up the bulk of your time spent in that zone just above threshold. But training at 105% to lift FTP? I think there are better ways to achieve the adaptations you’re looking for without incurring as much fatigue.