I have tried both methods for quite a long time (3x 11 minute casts for each method) and preemtively casting fire III after your last fire I resulted in the most DPS. It's not that surprising to be honest.
If I'm not mistaken, this is why:
My fire III hits for 1100 instead of 850
In my method it's basically: Fire I (750 damage) - Fire III (1100 damage) - Blizzard III (450 damage) - Thunder II 160 damage - Here the GCD is triggered after the Fire III (which always happens after you cast Fire III after Fire I)
In the second method you go: Fire I (750 damage) - Blizzard III (450 damage) - Thunder II (160 damage) - Transpose - Fire III - 850 damage (this is where your GCD kicks in)
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2460 damage my method
2210 damage the second method
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Basically, in both rotations there is a GCD where you can weave something in between. Am I missing something here?