Don't know but i wonder since i'm kind of in the same boat..
I would guess you get better or even results with summing within the Allen & Heath with something good on the stereo buss, like Portico tape emu or compression, or whatever you like.
let me explain...
A reason not to mix on a live desk like A&H is probably the slightly higher noise level right?
If you take multiple channels out of the A&H and sum it in a folcrom you would have a certain db drop and need to reamp it.
Now if this slightly higher noise is up in those channels you're gonna amplify that now too..
I guess this wouldn't be all that bad but i think the best way to connect to a focrom would be direct from your converters.
my 2cents
I do think A&H makes nice mixers by the way.