Well, it is still two gcds isn't it? It is more like 240 per cast in the build up phase. The potency of the blm spells is far higher here? I.e you would still start your third cast at 5 seconds and the blm would not be far behind. Honestly, the potency of the rdm skills look a little low to me (could be completely wrong as I have not got to 70 yet or do any actual testing beyond simply looking at the potency of the skills).