Today was the day to finally get around to replacing my 863's with 864's. I usually pack a LOT of weight and I bang the arse end on a regular basis.
Background:
OME 863's are "good for" 200kg additional weight over their standard spring, that equals 440 lbs over here and the 864's are good for a whopping 400kg (880 lbs) over standard.
As the tongue weight of my boat approaches 300 lbs and we typically stuff the back of the vehicle with 2 gigantic ice chests and tools, ETC and I pack additional fuel, the 863's are marginal at best.
Findings:
Ground to bottom-of-rear-flare measurements netted a gain of 1.25 inches of lift OVER the 863's........
In order to see how much weight it would take to get the vehicle back to where it was with the 863's I started stacking full .30 caliber ammo cans on the tail gate.....
It took 5,640 rounds of M2 Ball (430 lbs) to drop the vehicle back down that 1.25 inch gain.
Interesting. It takes about 200 kg to get the 864's down to the height of the 863's, exactly the amount of additional weight the 864 is supposed to handle....
The top photo is my "standard load" which is about 50kg in the back. This is my drawers, CO2 and my standard tools. In this photo the main and sub tanks are full.
The second photo is with 100kg additional weight in the back and the third is with 200kg additional weight.
I am not running packers at either end.
I think the 864 (my set-up) may be considered "extra heavy" and the 863/J considered "heavy"
What you referred to as a "J/2.5 heavy" combo may be an 850J/863. This particular combination yields a fairly "level" truck with a moderate rear cargo load. An 863J vs an 864 is about the same unloaded height as I mentioned above. So this can be a bit tricky to achieve the stance you are looking for.
If the 850J/863 is a bit too level, the 864 will really raise the rear fast and an 863J may be a better choice as it will compress faster than the 864.