Of the BiS sets listed by the balance, two of them are 2.5s GCD (dead spellspeed) and one is 2.48s GCD. In the last tier, the theoretical best was a 2.48s GCD set. RDM operates well with low spellspeed due to their mana economy (which at ridiculous spellspeeds can get a bit unwieldy) and with how cast>dualcast operates. A lower spellspeed means a longer GCD, which means more time to move between casts. Personally I ran TEA with a 2.5s GCD set, but I am most comfortable operating between a 2.48-2.5s GCD on the job.