stuck a Prowin in my Star UMP & went from 300 to 350 fps, so expect an fps increase + better fps consistacey.
One more thing you must remove the top frontmost screw from the gearbox to get the hopup lush with gearbox.
Aleast I did with UMP.
AEG spring diameter = 15 mm
VSR-10 spring = 10 mm
Lylax APS spring = 12 mm
PDI SS 2000 , 3000 , 4000 , 5000 springs = 13 mm.
hope this helps
PDI VSR-10 springs = 11 mm