Rick -
Another set of data points, with a set-up similar to your truck...
I'm running OME 850/863 with 20mm packers on each front corner (for leveling purposes), ARB with Warn 12K & dual batteries. On the rear, a Kaymar with a single swing gate w/tire (a second gate to be added...), and OEM subtank (I estimate this to add approx 125 lbs, full).
My caster with OEM bushings (& 31" tires) measures -2.20 +/- 0.05 deg, as described, with no additional cargo weight on board except a tool box... more of a negative caster than you might expect for this lift. The only explanation I can think of for the excess is that the caster was off to begin with for some reason (unfortunately, no 'before-and-after' measurements available).
I spoke with Christo about the caster correction plates, and he said that measurements on various trucks netted an apprx average of 4 deg of caster correction (I suspect you are already aware of this). Seems like the best approach while still using OEM bushings.
Just a few thots...
Cheers, R -