Fire 3 > Fire 2 til you're done > Flare > Transpose (which puts you in UI, for those of us who haven't been paying attention) > Fire 3 > repeat

With spell speed in excess of 500, your Fire 3 will cast before the mana tick in the UI segment, putting you at 900~ mana instead of 2000~, and you basically just go immediately into Flare instead of Fire 2. The issue is, it's not completely reliable, so frequently you'll find yourself casting Fire 2 with 900 mana, which is bad, or casting Flare with 2000 mana, which isn't good. Also, I'm pretty sure going Transpose > Fire 3 > Flare is going to force you to wait for the 12 seconds of Transpose.

It's a problem. Not an insurmountable one, or even necessarily a major one, but it does exist.