A sample size of one data point is included here for your consideration.
While the Glasair Sportsman and RV10 are obviously different aircraft, one Sportsman owner had the WARP retrofitted to his aircraft. He reports no change in speeds, and just a touch over 50lbs reduction in useful load. In that particular aircraft the larger impact is the aftward shift in empty C of G, thus potentially having a greater limiting factor on cargo carrying capacity as the aircraft may run out of C of G range before it runs out of useful load.