You're confused on at least two points.
[1] the Aussie lift instructions recommend 19mm (3/4") of strut travel in the knuckle.
My travel is 1.25" (32mm). Your reported mfc travel is 1.5" (38mm). Note that 6mm is not a big delta in ground clearance, which is why any off-roader lifts their vehicle in the first place. I have 17K+ miles with my lift in place. Not sure where you get 18mm.
[2] You are confusing clamping force or load and clamping pressure. The clamping force is the torque load on the knuckle bolt, 83 ft-lb for a 12mm, grade 10.9 bolt. Note that Dave Thomas incorrectly sites using a 100 ft-lb torque. At any rate, the torque load is constant no matter if there is 2 inches of strut in the knuckle or 4 inches.
Pressure (P) is force (F) divided by area (S). The surface area of a cylinder is 2*(pi)*R*h. For argument sake, let's say the strut dia is 2". The height of the knuckle is 3.5" (on average, it's actually tapered on the bottom). Thus, the engaged S of an OE strut is 22 in^2. If the knuckle is lowered 1.25", then h becomes 2.25" and S=14.1 in^2. Similarly when the knuckle is lowered 1.5", h becomes 2" and S=12.6 in^2.
Back to the clamping pressure, P=F/S. Use any force number you want because it is constant. For argument sake, let's use a round number like 100 lbs. So
At OE, P=4.55 psi
At 1.25", P=7.09 psi
At 1.5", P=7.94 psi
Do you see a trend there? Increasing clamping pressure as surface area decreases. So what's the argument for using a dead weight clamp ring? Oh yeah, so you can charge more for a lift kit. How much was that again?
Note that there is a critical h value where the entire strut is above the torque bolt. The force is now no longer uniform. It is not necessary to go into those equations. There is another critical h value where the CV angles are too sharp for the OE design. Each of these cases is unfavorable.
Also note that 83 ft-lb of torque on the knuckle bolt does not equate to 100 lbs load. However, I don't need to drag that equation out to prove my point.
Anyway, your lift is not better, it's just different and more expensive. If 6mm means you have a better lift then I give up and you be the winner. Whatever lift you have just enjoy it and go Jeeping.
Happy trails,
unofficial Jeep Field Test Engineer
References:
[1] Dave Thomas,
http://www.justaskdave.com.au/kl-cherokee-lift/
[2] Robert K. Emmons, Jr.,
http://www.jeepforum.com/forum/f428/there-s-hope-off-shelf-cherokee-kl-lift-3655881/