You don't need to have the tubes fully filled with Slime or equivalent to make the fluid effective. You only lose it in very small amounts each time you have a self repairable puncture.
I live in a region filled with Rubus ulmifulios (translate that to LOTS OF THORNS) and rode 2 years with the same 29er tubes with Stans fluid. Never had a flat, but frequent inspection revealed at least an hundred of small punctures. So I guess it works pretty well.
Also, regarding the weight penalty, consider that to the slip/low drag between the fluid and the rube, the time where wheel weight matters the most which is acceleration, the mass of the fluid will stay at lower speeds than the tube, therefore lessening the effects of it's innertia