Transpose is only really when you are lower levels as when you get higher you can just use Ice 3 and Fire 3 which brings you straight to Umbral Ice and Astral Fire respectively.
It still situation dependant as well because casting ice 3 for instance takes longer then if you would transpose but the tradeoff is ice 3 fills up your mp while for instance transpose> ice would require multiple ice casts to fill your mp. Also convert is great when you get that ability as well.
I have not really done testing on casting times but know that the dps section of the forum many have done so