While I haven't received the MarcMooga's board I did 3 more tests using exactly the same boards as yours with exactly the same MegaDrum and USB firmware. The test is done on a laptop with built-in sound card and ASIO4ALL and using Cubase.
Time between I hit a pad and the sound signal at the headphones out jack using USB MIDI:
MegaDrum-via-USB2-vai-Cubase-via-ASIO4ALL10ms-latency.png
It is 18ms.
Time between I hit a pad and the MIDI signal at the MIDI Out:
MegaDrum-to-MIDI-latency.png
It is 4.5ms
And just as the reference I used my Novation Remote 25 keyboard to measure time between MIDI Out signal out of the keyboard and the sound signal at the headphones out jack:
Novation-MIDIOUT-via-Cubase-via-ASIO4ALL10ms-latency.png
It is 14ms
As you can see out of 18ms for the total latency over USB MIDI, 14ms are contributed by USB-MIDI driver/OS(Windows 7 64bit)/Cubase/built-in sound card with ASIO4ALL. ASIO4ALL (if it can be trusted) is configured to have 9ms latency leaving 5ms to USB-MIDI driver/OS(Windows 7 64bit)/Cubase.
I doubt very much I will find anything different once the MarcMooga's board arrives since it is identical to the one I did the test with.
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