I've tested this again an I'm now very confused by the results. 
I've set my MP-201 to "full blast" meaning that the MIDI output is a fast as possible, with no traffic restriction. ("midi governor set to "0")
Here is what I've found:
MODFACTOR -> Q-Wah -> Vintage Wah : works perfect! Same as with an analog expression pedal…
MODFACTOR -> Q-Wah -> Mumbler: very jumpy response.
Then I've realised that on the Vintage Wah there's not a single control with a minimum and maximum value??? I see that "Q-Intensity", "Depth", and "Bottom" are mapped to the expression pedal, but although there's 2 dots there's no arc and moving the expression pedal wil not change anything on the display.
I really don't understand what I'm seing there on the H9 control app??? What's the expression controlling if nothing change on the display? And, what are the meaning of those 2 dots on some control if you cannot change their position? And, the circle are greyd, once you unmap the control it is impossible to map it again, although that doesn't change anything… 
Then I does indeed works perfect with the midi expression pedal I have…
I've then unmap all the knobs of the "mumbler" but the "bottom". Leaving only a single pot mapped to the expression pedal. Big difference! This presets now works very well with my midi pedal.
It appears to me that in order to be able to use midi for expression you have to limit the number of controlled parameters (pots mapped to expression) to a minimum.
At this point I should say that the results are "acceptable" to me. Meaning that I believe that I could use midi to control expression succesfullly with most presets, maybe sometimes by reducing the number of pots linked to the expression.
AT this point I'm unsure if I changed anything else on my H9 which could have improved the midi response. Any comments about what you're experiencing would be appreciated. If possible with detailed setting your using.
EVENTIDE: I've mentioned in a previous post the fact that the led ring doesn't update with midi expression received. You said that you added this to your "to-do" list.
I would like to add to this that the display on the H9 Control software doesn't update it's "hot bar" (?). I'm talking here about the "slider" below the pots, on top of the bottom switches. Just a little annoyance, but I wanted you to know.
LA
