Does anyone know if there's a difference in these heads? I would imagine the 20vt head would have larger ports and valves? Anyone ever stuck a 20vt head on a 20v na if so?

20V from Coupe is the same. The one from 2.4 Kappa or Stilo has bigger valves. But still awfully small ports )))

Cheers

Can enough material be removed from the 2.4 head to make the ports around 75% the diameter of the valves?

It can be done yes. But you may loose lowrev torque cause of lower airflow speed with these channels. It's much more important to upgrade valves - back cut etc to make a slot between valve and seat more productive.