Despite what other might say I towed with a Hensley for 5 years or so. I had a 11K GVW TT loaded to 9.5-10K most of the time. I had custom genset platform welded on the rear. I carried a Subaru 4300IS genset on the platform.
THe platform and genset added about 375-400#'s to the rear of the TT
The weight of the platform definitely affected the performance of the Hensley.
I took a few test rides like you mentioned. When my tongue was light I could feel the instability in the trailer.
In the end I shifted my cargo around.Using a Sherline tongue scale I was able to keep my tongue at 13% or 1250-1300#.
With the proper tongue weight my combo was rock solid. I am a big believer in the HA and PP however TW did affect the performance of my HA.
I think the PP will go a long way towards resolving your issues but I don't think you can ignore the light tongue, assuming the PP will take care of it.
I'm not sure if you have made a trip to the scales yet. But you need accurate real data to get a true handle on your combo. A Sherline scale would also allow you to precisely dial your tongue weight in.
