Yes you can use them on heavier rigs. They don't work as good particularly if you don't have at least half the shaft showing in respect to the older ST version. The STX version works better in terms of handling in the same scenario because of a redesign of the tube system that circulates the oil during travel.
Once you hit 6 k even the manual SUGGESTS running more than one strut per corner if the vehicle has poor handling charcteristics.
I have had Mark Jensen the owner of ORI, at my home for a weekend. We talked extensively about them and tuned on mine. Including him making me the first pair of remote piggy back reservoirs to help the situation on my Jeep. To further explain, when less than half of the shaft is showing there is less nitrogen that you are riding on when the strut is compressing before the strut hits the "oil" stage of compression. As a result, the vehicle will ride rough because it moves right through the nitrogen sitting on top of the oil and hits the oil which result in a hard ride. More nitrogen on top of the oil means softer ride, less rougher ride. This is made worse with a heavy vehicle because you have to run high upper chamber pressures to get your ride height corect. Running only 40-45 lbs in my bottom chambers, almost the bare minimum, I still have to hit 300 lbs or over in my front upper chambers to get the correct ride height on my Jeep. Since the bottom chamber controls stability you might be inclined to charge it higher. When this is done the upper chamber will have to be increased exponentially even more. In fact, if I charge the lower chamber to 80-90 lbs then my upper chamber requires pressure in excess of 400 lbs to get ride height. Thankfully, making tuning changes with the pressures is as easy as letting nitrogen out versus having to switch springs like a coilover.
The piggy back reservoir, at least in my case of a heavy vehicle with the bare minimum of shaft showing, attempts to provide additional nitrogen in the upper chamber to make the ride softer. As there is an increase in volume of nitrogen within upper chamber of the strut, but not necessarily an increase in pressure which makes the situation worse.
So it would not be a surprise that what you saw was a vehicle that looked very stable. My Jeep, 5700 lbs, rides very stable. But, that is the problem. It is to stable because I have right at the bare minimum of allowable shaft exposed. Thus, it rides rough particularly on slow stuff. Going fast is better as the strut goes through the nitrogen stage of dampening quickly into the oil stage cushioning the hard hits better.
a pic of my rig, checkin for clearence issues,,
