that's a really dumb theory you have there, 20% decrease in mass doesn't mean 20% increase in velocity.
things don't work like that, it always have to follow a formula. We'll keep it simple, you can find the difference in velocity between 0.20g and 0.25g using 2 physics formulas
vf(squared) = vi(squared) + 2ad
F = force applied
m = mass
a = acceleration
vf = Final velocity when leaving the barrel
vi = initial velocity before force being applied(so in this case, zero)
d = distance traveled between vi to vf
did some calculations and here are the results(In the calculation, I'm assuming that it fires 400FPS with 0.25g)
I also neglect any friction force between barrel and BB during barrel travel, and hop up effect.
From my chrono results(and yours too) shows that the difference in velocity between 0.20 and 0.25g IS roughly 40FPS give or take a few FPS (because of hop up/friction).
so no, 20% mass decrease in mass does NOT equal 20% increase in velocity.