Hi Harry

I have flown the Guardian in HH on my MB339 for testing. I had it in low rate mostly a non event, at high speed it just kind went where you pointed it if the plane could hold it aerodynamically.
I have a Gen 1 Skymaster MB339 that is pretty heavy 43 lbs Dry approximately so with the PST 1300R it doesn't hold much for long lol.
Our field also has a very tight pattern so its hard to do long legs to see how some of the HH worked.

It made 4 points easy, I was also able to land it in HH mode without issue, as you have seen as long as the gain Is low enough it won't drive the plane to stall.

I don't think that It will be super successful in killing small trim changes, I believe that over time the plane will still drift a bit especially as you may not be generating much acceleration, or angular acceleration in that situation.
it should dampen big changes like putting flaps down at high speed ok, but after things settle down I still think and out of trim bird will feel that way.
(HH really reduced the amount of elevator I had to hold during the 4 point so for that trim change it did work well)

During normal high-speed flight I felt the plane felt notchy in pitch, It was hard to put the plane in a level flight attitude that I was satisfied with. It was always climbing or diving slightly

Here is my Mig-21 from global 6 oz tail heavy during its maiden Guaridan in normal mode. https://www.youtube.com/watch?v=6FLoY3bz5i8
Would have been real fun without the Gyro lol.

I liked HH in some situations it would be nice if we could turn it on and off like the IGyro.

With the same plane I also tried HH with a Futaba Gyro on pitch only to try and get the MB339 to hold a commanded Angle of Atttack, didn't work too well the plane kept wanting to pitch more and more up it did not hold an attitude as I had hoped. It was to
scary to continue with that line of testing so I stopped that experiment.

Been playing with this unit pretty neat so far, have not put it a jet yet.
http://www.bluelight-tech.com/