Comparing stratolauncher payload to Falcon Heavy’s gross weight at takeoff is not really a helpful comparison. FH has a 60 metric tons to LEO max payload, roughly 5% of its total weight. I’m sure someone has done the calcs on the theoretical potential of the Stratolauncher, but it looks like a Pegasus can get 1000 lbs to LEO at a total weight of 40,000 lbs, so same 5% ratio. Seems to be in the same ballpark, and Stratolauncher (would?) use a fraction of the resources.