So apperently someone linked to the dev team says transpose > blizard IV, never using

[Ferth]

My math could be wrong, but I am coming back with using transpose as a 100 potency loss (5 potency per second lost over 22ish seconds) in a standard 4f4 rotation using no procs or abilities. I don't have the mp necessary to cast an extra fire 4 with transpose unless you get the global tick conversion just right, with that extra fire 4 you end up at a .5 potency per second loss over 25ish seconds.

It is a negligible difference but unless someone has different math it looks like, overall, transpose is a slight dps loss. The only thing that might make it more beneficial to use transpose is if you have inadequate spell speed and are having trouble casting thunder in every umbral phase.