Trying to do my own research using info supplied here and other sites, my thoughts are in closed loop the tuners are not able to change the AFR rich enough to actually cool these motors because of the HD O2 sensors. I think that is why you are seeing the auto-tune systems using different O2 sensors. Both the PCV auto-tune and the Thundermax auto-tune use "wide band O2 sensors".
Tread lightly on this subject though it can put you in the line of fire, especially here at CVOharley.
That's a good summary. The SEPST will not allow the AFR to drop below 14.3 without dropping out of closed loop. You can try to reduce the range of cells in closed loop by lower them below 14.3, but if you take the entire map out of closed loop, the O2 sensors will kick out a trouble code since they are essentially turned off.
Closed loop at 14.3 is not necessarily a bad thing (although it's clearly lean in order to meet emissions standards). However you can still have a very throttle-responsive bike that performs outstanding even though some areas of the map are in closed loop. The areas in closed loop are generally at idle and in the low load areas of the map generally below 4000 rpms (although this varies based on the engine map template you start with).
The higher the performance of the engine, the smaller the area of the map that is within a closed loop "region". This closed loop region of the map does have the advantage of enabling the bike to run better in different weather and altitude conditions when compared to altitude/weather under which the bike was originally tuned. Also by having the cruising range running at 14.3 you will have an outstanding gas tank range combined with outstanding performance when you "jump on it" which pushes the ECU into the region of the map that is not restricted to closed loop performance at 14.3.
Hopefully that wasn't too confusing and is helpful interms of explaing some basics for the SEPST anyways. Lot's of other tricks I've learned along the way that have really made the bike perform outstanding vs the original map that came from the tuning at my dealer. Lowering the size of the injector is an easy method to reduce the heat of the engine. Just remember, if you lower the injector size and then attempt to use Smarttune to tune the bike, the software will attempt to reverse the your efforts in an attempt to get the tune back to 14.3 (in the closed loop region).