Been there done that. I have seen 7:00pm Fantasmic and then made the 9:00pm Illuminations show. It really wasn't that bad. We took the boat from MGM to the Yacht/Beach club and walked from there. We have also done it another time when we walked the whole way. That cut the time a little tight, but the boat was nowhere to be seen. The frustrating thing about the boat is that it stops at the Boardwalk prior to EPCOT. It is shorter to walk from the Yacht/Beach club. And of course there is always the bus, but this drops you off at the front of EPCOT leaving you with a long walk to Illuminations.
But then again... you aren't going to Illuminations... you're going to Magic! That is a little harder since the MK is farther away and you have to deal with the TTC.