Hey, just so everyone knows incase you dont, a gas gun will have more energy efficiency with a heavier bb. My short answer to the question, with a 550fps with .2g bb, when you put a .43gram bb, my guess would be, it would shoot between 425-450fps, having a higher energy than if you were to shoot .2gram bb.
A couple weeks ago I did testing in my basement, Here are my results
Gun: KJ M700 takedown
Airs supply: Custom Made CO2 rig, with adjustable pressure
Chrony: Styrak's Madbull Chrony.
At 90Psi (about what propane will shootout sometimes)
.2g bb at 505fps
.28g bb at 470fps
.2g bb at 550fps
.28g bb at 505fps