The only hint I can give you about getting the RTG version running in fullscreen is this from another user as I've never managed to get it working myself
To get the games playable I had to switch my WB-screen to a 320*240 8bit resolution, since SCUMMVMRTG seems to take the resolution used by the WB.
And you are correct, getting it to run in fullscreen is your best bet for more speed.
We did find that certain games (eg DiscWorld) seem to play MIDI better than some other games. If I ever get the time and energy I might port a faster MIDI engine over to ScummVM which could be used in both the RTG and AGA versions.
I cannot do anything else to make this RTG port faster, I only did this RTG version because it was very easy for me to do using the 68k SDL, my real interest is in AGA. Unfortunately the old 68k SDL is no longer maintained which means that this port isn't ever going to get any better
BTW Amiga Doom does not use MIDI (I should know, see the DOTT thread) so that's not a good comparision. I think the problem here is that the modern versions of ScummVM are targeted at more powerful hardware and so they don't have the same issues as the Amiga versions with things like MIDI playback and 16bit overlays.