Here is my idea. I have no hurtful pride here so laugh at me all you want. I am thinking about modifying the BEGI FMU I have.
I feel it is not tunable enough as I am always on the too rich for good power side. Consistent 11.3 to 10.3 AFRs' on boost regardless of how much I back it out. The only thing I seem to change is the onset of fuel enrichment not the amount of it. Once it starts it just goes really rich.
My diagnosis is that the bleed off system of the FMU is too small for our cars to control the pressure rise with larger injectors.
My solution would be;
Drill and tap the hole for the bleed off valve and install a larger needle valve.
Use a 1/4" spring loaded check valve in the system instead of the little plastic one it comes with. Cracking pressure is .5 psi.( I have already swapped out the little check valve for the one off of the thermo-time switch vacuum system. It flows much better)
Spend a little time on the Dyno and get it tuned in the 12.5:1 AFR range.
So now, please tell me all the ways that this won't work. I really do want to know. Thanks.
Alright, just to prove I'm crazy...
Another way to lean out the mix is to increase the boost. 
Have you tried running without the FMU?
Yes, your solution should work. You're looking for a reduction of pressure in the chamber of the FMU so, if you can bleed off all the air that's coming in you will get no increase in pressure.
Another option is to insert a micro flow control valve in the signal line... essentially an adjustable restrictor... no drilling/tapping or altering the FMU. Mount the flow control valve in the cabin and adjust on the fly... or just turn the boost up
Have you tried running without the FMU?
Yes, your solution should work. You're looking for a reduction of pressure in the chamber of the FMU so, if you can bleed off all the air that's coming in you will get no increase in pressure.
Another option is to insert a micro flow control valve in the signal line... essentially an adjustable restrictor... no drilling/tapping or altering the FMU. Mount the flow control valve in the cabin and adjust on the fly... or just turn the boost up
O.K. lets clarify one thing. I turned up the boost. Or more accurately, I jacked up the boost. 
To be even more accurate I shimmed up the boost. (about 3/8" more spring compression at rest)
How much more boost do I have? Dunno. The boost gauge I ordered has not arrived yet. I am sure at this point that UPS just hates me.( too long of a story to get into here)
So anyhow, back to my original point.
So far you've given me practical reasons and alternate scenarios to avoid modifying the FMU. All good info for sure but not really what I was looking for.
What I am attempting to achieve is to infuse a larger window of adjustability into unit.
Sweeney your suggestion of putting the restriction in the pressure hose is certainly the more elegant solution.
And just so you know, I will be on the Dyno tomorrow morning at 10. I will have revised AFRs' at the new higher unknown boost level. I am starting to think I should change my name to "ill prepared bastard"
To be even more accurate I shimmed up the boost. (about 3/8" more spring compression at rest)How much more boost do I have? Dunno. The boost gauge I ordered has not arrived yet. I am sure at this point that UPS just hates me.( too long of a story to get into here)
So anyhow, back to my original point.
So far you've given me practical reasons and alternate scenarios to avoid modifying the FMU. All good info for sure but not really what I was looking for.
What I am attempting to achieve is to infuse a larger window of adjustability into unit.
Sweeney your suggestion of putting the restriction in the pressure hose is certainly the more elegant solution.
And just so you know, I will be on the Dyno tomorrow morning at 10. I will have revised AFRs' at the new higher unknown boost level. I am starting to think I should change my name to "ill prepared bastard"
