Why are you saying anything if you're not capped at 90. The whole reason why Fire IV was buffed to 310 is because Paradox was too close of Fire IV DPS and Paradox Mage became a thing. Paradox Mage is a meme that shouldn't exist like Dragon Kick spam Monk right now.

You shouldn't calc things as pure potency but the potency per second.

310 *1.8 from Astral Fire is 558 Potency for 2.8s. 199.28 PPS.
500 Potency from Paradox in a 2.5s is 200 PPS.

With BiS Spellspeed
558 Potency for Fire IV in 2.47s = 225.91 PPS
500 Potency from Paradox under 2.2s is 227.27 PPS

Back in ShB, I recalled people skipping Blizzard IV because Fire I wasn't great but because it turns into a spell that deals better damage than Fire IV in which, can be used in both your form, you probably should stick to use Fire I until you're level 90.