Despite being a big feature for RTX5000, RTX Mega Geometry actually benefits older hardware much more than new.
A 9800x3d paired with a 5080 or 5090 sees basically no performance increases with Mega Geometry. But pair a 3600 and a 2080ti and you see 15% higher framerate than before. This higher framerate applies when pairing a strong cpu with a weak GPU but especially applies with a weak CPU, the weaker the CPU the higher the frame rate increases from having mega geometry on
So overall despite Nvidia saying Mega Geometry runs best on RTX5000, it looks like gamers with older CPus and GPus will see the real benefit