When they say massive GM caliper, I am assuming they have the 2.0" diameter pistons that come on the 1999 and up Silverado/Sierra as that is what they look like in the picture. If that is the caliper, they will be cast iron, but because the size of the caliper pistons (2.0" diameter), it should perform substantially better than the Corvette (1.59" diameter pistons) or Camaro (1.77" diameter pistons) twin piston calipers. Larger pistons mean better clamping forces and cast iron usually does not deflect as much as an aluminum caliper. The calipers will weigh almost twice as much (I am guessing) as the Corvette or Camaro calipers.
I would get more specs on the calipers, rotor width, etc before dropping $830.
If they are the 1999 and up Silverado/Sierra calipers, it should stop great with larger 13" rotors and it should have better braking abilities than the 14" rotors and 6 piston calipers combinations. That is because the 6 piston calipers have a smaller piston area of around 4.25-4.0 square inches per caliper versus 6.28 square inches of piston area per caliper for the Silverado twin piston calipers. That is about 30% larger piston area. Larger piston areas "usually" means more clamping forces regardless of piston count.