that if a gun is "over sprung" (spring too strong, too much preload, heavy top hat, or whatever) with a normal pellet, the heavy pellet will show a greater velocity gain proportional to pellet weight.
Way back when I used to use the heavy wire Maccari SteelGate springs in my R9s the velocity of CP heavies would increase by about 50 fps but the CP lights would only increase 30fps in the same gun over factory velocities. When I had a .20 barrel on my R9 I used the SteelGate spring to try betting 800fps with Beeman FTS pellets usung a factory seal only to get about 790fps. Once I accidently left the gun cocked for a two weeks and even though the gun cocked easier with the fatigued spring the Beeman FTS would fly over the chrony sensors at 810fps. Definitely a case of over driving the pellet when the spring was "unstressed".
Anywhoo......just food for thought here.