What's the limiting factor when designing to a constellation? Otherwise, what's improved about the chip or firmware to be able to move from 256QAM to 512QAM?
signal/noise ratio is a big one and RSL level. There are even 4096QAM radios now but they need a signal like -51 to operate at their full modulation. Otherwise they ACM down to 1024 (around -62), 256 and then 64QAM.