Peter,
The MIDI used to be about the same relative volume as the audio output, but since the bug, it is about 1/2 as loud as it used to be.
My external sound module has not changed in volume since the bug was introduced.
I have tape markings on all the volume controls so I'm 100% sure that has not changed.
Another workaround that I've used it to "render with options..." and select "normalize" on all tracks.
Then when I bring the tracks into Logic the MIDI tracks will actually be a bit louder than I want, but better a bit louder than too soft.
There were a lot of complaints on the Apple forums about this problem a year or two ago, and I'm told that 10.8 is still low when compared to 10.5, but I guess folks have just learned to live with it.
Apple did acknowledge on one of my bug reports, that it was a known issue, and it would be looked into.
Maybe they just wanted to shut me up.

- Jay