That looks low to me too. When you pick up the rear, does it lift easily for several inches until the suspension tops out? If you've got too little preload, you lose a lot of travel and will bottom out easily. My sled has torsion springs like you mention with a rectangular block you can turn to increase/decrease tension, but some newer sleds use a spring-shock instead of torsion springs. With a coil-over, you'd just turn the adjuster until you got it where you needed it, and an air setup should work similarly but with more air pressure. I'd suggest adding air until you reach the FSM recommended level, but might not be a good idea if you're already at the limit. On a mountain setup, the rear has a far more pronounced effect than the front (referring to the skid, not the skis), and on mine cranking up the front spring won't lift the rear a bit. Yours looks like a coupled setup though, so maybe turning up the front spring lifts the rear too – on mine you can wind it as tight as it'll go and it won't lift the rear.
02 Polaris 600 RMK
Project sleds: 89 Indy Trail, 94 AC Pantera