I think you have most of the differences pegged. Moving the -3dB point to 20 Hz instead of 28 Hz is probably the biggest as you already noted.
I'm not 100% sure, but depending on how you interpret the response curves the EP500 may also react more gracefully when it does eventually reach its limits. I don't know exactly what the DSP does when it decides that either power or speaker travel are about to go too far (ie does it turn the volume down across the entire frequency range or just roll off the lowest notes) but either way the result has to be more pleasant than the loud GRUNK that even the biggest subs make when they hit the wall.
Bottom line is that the difference between 350 and 500 seems to be a number of different areas. It will be interesting to see what Axiom does next with the sub lineup. Heck, it's probably worth driving to Dwight just to find that out
