Look I have a similar set up in the belly of a full scale 727-200....no pumps just bleed air supplying 10 psi of head pressure to what amounts to 2 tanks one in each bag pit.....pressure remains constant.....as the fuel is transfered into the nbr 2 tank the volume of space increases..(transfer from 1 tank at a time only) pressure has to decrease and the fuel transfer slows to a crawl....
In case your wondering the volume increaseing is off set in flight by the outside air pressure dropping..you transfer as fast as you burn
now in this case with a 40 LA...the 12 oz tank seems a bit large...8 oz seems to be more what I remember for a 40 size....so look at this at full throttle.. lots of pressure..as you throttle down the supplied pressure drops, head pressure decreases and volume is increaseing(fuel burn)...we as RC'ers never get the altitude for an outside pressure drop....if the engine can't produce the volume needed to fill the void transfer of fuel will stop just like a fuel line coming off....
A header tank will only increase the volume....
remember this is only a possibility...but I'd try and plumb in an 8 oz tank and run that on the ground and see what happens
as always Good Flyin