I installed one of the old M-series Burandt kits in my rear skid shocks. I am 260 without gear. My biggest gripe with the stock setup was I would get a bucking sensation from the rear track shock over big single bumps and rollers if I was not on the gas with the weight transferred to the rear. I believe I pulled one shim out of the upper part of kit's suggested valve stack on both the front and rear track shock to stiffen them slightly to start with. Rear is pretty good now, but the front needs a little stiffer high-speed dampening still. I think this is the most economical option for a decent upgrade.
The Tom's snowmobile SIK kit seems to be working well. I think that is the route I would go if I was doing it again. Replaces the front shock with a new generation float and adds an evol chamber to the rear with high-flow valving pistons and the new Fox bleed shim valving.