I am getting about 120 miles to a tank. Will a remap improve this? I don't expect miracles but how attainable is 30mpg?
That is particularly bad. Mine usually averages 200 miles; 250 when I'm just going up and down the motorway all day (I was commuting to Stoke from North Wales); and about 180 miles when I'm belting up and down the back roads. - I'm not on boost all the time, but I'm not driving like a nun either.
A remap will definitely improve economy (for the reasons mentioned above), but you may have other problems; a poorly adjusted TPS or dirty MAF maybe? Perhaps your MAF needs to be re-earthed.
WRT to the dump valve, I'm not sure how much difference it makes. Doesn't the injector duty stop when the TPS is at the closed position? - I did have someone tell me that the reason you can melt pistons easily is dump valves... because it gets lean. That made no sense to me (but you do Jon), so I ignored everything else he had to say about these engines.
Perhaps it doesn't go rich for very long; I don't really see that recirculating it makes any difference because you've closed the throttle butterflies, and that's what caused the dump (or recirculation) of the air.
Obviously I'm happy to be set right on this.