Is there a lot of work involved to use a non AT G2 grip set with the G2 reel seats? Or is it just easier/better to keep it all AT?

27 mm is the OD of most casting trigger reel seats to 20 mm, there are wierd ones like the Fuji SK2. Most cork, EVA, etc. grips are also that size so they match up. The 17 mm Aero spinning seat also uses this size grip, the 16 mm Aero seat is 1/10 inch inch smaller in the pocket the grip slides into so you need a smaller grip.

Here's one with as received MHX Winn grips