I play with 250ms and I'm really surprised yours is much worse now; 400ms is awful. I really hope it's just down to teething problems and it settles down to a more manageable level soon so you can play the job you want.
Even with my slightly lower ping, the latency is noticeable for BLM (you can't even queue the IV spells until they're 'unlocked' by the two buffs they require, which means that in a laggy situation you have a harder time keeping your rotation working smoothly) and for MCH, as procs will register more slowly than is ideal. It's a shame as I felt that BLM was quite resilient to high latency until the new mechanics arrived.
