This is completely off-topic for a simulationcraft thread.
There seems to have been a lot of discussion about the viability of Fel Flame but no real conclusion, I have taken a look at this over a few evenings to see if I can see anything specific and it does look like it would be better.
disclaimer: no real figures logged and this is for affliction only.
refreshing UA and Corr costs 2x GCD and the cast time of UA.
refreshing UA and Corr by FF costs 2x GCD and also adds 8k damage/16k crit
gain of 1.5s for FF and the added damage, loss of 2s of Corr duration which at level 85 is ~4-5k tick maximum.
refreshing UA or Corr as a hard cast will only put it back to the original duration of the spell
refreshing UA or Corr using FF x2 will refresh both by 16 seconds, this is equal to the original cost of UA and -2s on Corr.
refreshing UA or Corr while on the move constantly will stack the debuff length of UA to 25s and Corr to 29, this allows for MUCH more mobility.
Again, I don't have any actual figures but these look far more positive on the use of FF