If long haul APD was devolved to Wales it would also involve long haul via hubs such as AMS. Any hope that BRS had of getting a NYC or ME service would almost certainly disappear if CWL enjoyed a low or zero rated APD rate. CWL would then become the airport of choice if any airline decided to set up long haul direct scheduled flights from Severnside. Long haul transatlantic charters, say with TOM, might be more likely initially.
I submitted a response to Letsflycwl's post in the BRS thread part of which is quoted above. Broadly it agrees with Fairdealfrank in that transatlantic charters would be the most likely benefit if APD powers were devolved to Wales, certainly in the first instance.
I don't think that either BRS or CWL has any chance in the forseeable future of getting one of the MEB3 whatever the APD state of play. Turkish Airlines to Istanbul is far more likely.
Whether a zero APD rate in Wales would swing Turkish for CWL is arguable. Given the considerably smaller and less affluent CWL catchment (than BRS) a lot more passengers would have to be found with origin/destination outside Wales
As for NYC, BRS had a daily (4-5 x weekly in winter) CO B 757-200 to EWR for over five years until late 2010 but in the end the airline could not make it pay although it did eventually founder during the depths of the recession. On a purely commercial basis (as the BRS-EWR service was operated) CWL/NYC would not have a prayer but with an indirect public subsidy such as a zero rate APD and/or a direct public subsidy such as the Wales Assembly owners oiling the wheels (they'd have to be careful of not contravening state aid rules) it might just be possible, but I still wonder where all the passengers would come from.
There is no doubt in my mind that having the ability to lower/effectively abolish APD would be a great chance, perhaps the only realistic one, for CWL to mount a real recovery and then lay the base for further growth after a dreadful seven years (with only 2013 giving some small hope that things were getting better).