Ok so according to Wikipedia, Deluxe Paint IV 4.5 is the first version that supports 256-color modes provided by AGA.
According to my tests, DP 4.5 understands the 8-bit ("chunky"?) file format used by Grafx2 and Deluxe Paint 2E, the loading will succeed no matter the "screen size" it was saved in. When it asks if you want to change the screen mode, just answer no to stay in your current preferred resolution. The Page will resize accordingly to image pixels, no problem.
However! If Deluxe Paint 4.5 is started on a non-AGA machine (in 32 colors, anyway), the program performs a color-reduction of the image to an optimized 32-color palette! This means the palette order is not guaranteed to be preserved, and unused colors will be lost. For game/demo purpose, this is generally destructive so I don't advise moving files from Grafx2 to Deluxe Paint 4.5 non-AGA.
About editing <256 colors in Grafx2 : This is an idea. It would be useful for several file formats (for the C64, Amstrad CPC, Atari ST) If we can enforce an image to stay 16 or 32 colors, it would make sense to save in IFF as 4 or 5 bitplanes. Such files should be re-loadable in many OCS programs.