Would have to assume they shouldn't be fully compressed. Looks like the short spring is the softer of the two, so those would soak up the small bobbles down a straight while the longer springs absorb the jumps.
Softer in the rear would give you better forward bite. Allows the weight to transition to the rear. Considering the width of the tires and power of the rig, you would want a lot of weight transfer to the rear when accelerating or it will just spin the tires... as your kind of finding out.
That...
I run a set of duratrax hatchets on my e-jato now. Awful awful foams that flattened out after just a few short packs, but the tires held up ok as did the rims. I refoamed the rears with some spare foam I had laying around.
I found when I ran springs/oil that could handle a 5 foot drop without...
Not sure the diff will matter as much as throttle control, adding a bit of toe in on your front, softening up the shocks... my jato was a nightmare to drive when I had stiff springs on it.
Also, depending on the surface your driving on, you may need softer tires or a more aggressive tread.