The Silk Commission recommended that long haul APD be devolved to Wales in the first instance so it may be that is what will happen. The discussion paper to be published in the summer seems another way to delay a decision. Following a full scale review of APD devolution a couple of years ago the government put the whole thing in the too-hard-to-do basket. That changed with the possibility of Scotland voting for independence when all the main political parties made promises to devolve more power north of the border including APD in a late dash to avoid a break-up of the UK.
Now that Scotland will have the power naturally Wales wants the same. However, there is not the same urgency because Wales is not in the process of voting for independence, hence another delaying tactic to give the government breathing space to decide how things might be evened up in England (if there is a way).
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.
It's not clear if any other airport other than BRS would be affected with long haul APD devolved to Wales. It seems doubtful that long haul services from more distant English airports would migrate to CWL.
In the end it will be a political decision. Obviously the BRS ownership and management believe that markets should be left without government interference but the CWL ownership and management take the opposite view. Were the positions reversed so would be the respective beliefs. There is an additional ingredient with CWL in that its owner will also be setting its APD rates if the tax is devolved. Whether that will complicate matters in any way remains to be seen.