Apparently the atmosphere is 100 times denser between 550 km and 350 km (roughly 3.5e-13 vs 3.5e-11 kg/m3). It's still hard vacuum either way, but I wonder how that additional density would affect the current 5 year lifespan number. Perhaps full-size V2 satellites have so much propellant onboard that they can keep the 5 year lifespan even at the lower altitude.