It nerfs the damage though because you are trading 1100 for 600 due to the fact you can cast it twice for every once with your change.
There is only 10 seconds down time with the 25 CD when you don't have the DoT up as it stands.
Your change would mean there is 25 second of nothing after the DoT falls off.
So if anything the longer the fight went on the worse off you'd be.
I also misread your fell cleave change due to not having the original potency on hand and how you had laid out the other changes but it seems like a unnecessary change.
Also my math is bad because I don't do damage calculations ;-; so Pooky down there V probably explains it better!
