Boosting the velocity does produce a small change in volume, but the MIDI thru is still completely inaudible when any other tracks are playing. (When all are set to 90, I mean: If I turn down every other track, I can hear the MIDI thru.)
Just so you know and understand:
1. The MIDI volume (CC07):
One can think of it as the distance to the instrument, which is typically constant. A MIDI track should start with setting the instrument(s) to a volume that will not create distortions when I play a note as loud as I can.
2. The MIDI Expression (CC11).
This is really how the instrument is being played, meaning how hard the trumpeter blows, or how intense the violinist plays. This can affects mid-note volume. (It seems that for many drum-sounds, especially the short shorts, velocity is better to alter the dynamic volume.)
3. The velocity. This affects how hard the note is started, but does not change mid-note volume. For some instruments, it will actually not mean that much to volume, but will rather change to a completely different sound (for example some of the drum sounds in SD2).