That's a good question. Lotharek's page seems to imply that there's up to 14 MB of chip RAM but I'm that sounds a bit odd. I'm thinking it's actually 16 MB total RAM with 2 MB set aside for chip RAM. Lotharek mentions that his board is using Minimig cores so it should be able to use the AGA core as long as there's enough room on the fpga.....not sure about that.
MIST can't do 24 bit video. If I remember correctly it's limited to 18 bit, so colours would be wrong with AGA.