Rain of Fire shouldn't be buffed though. It's OP the way it is. If anything the damage should be REDUCED by 1.
What are you talking about? Firestorm costs 7 fire and against most decks is completely useless except against FFQ decks and rainbow creature spam, and even then its not completely effective. Atleast make the upped version of Firestorm do 4 damage instead of bumping the cost. (I respected Firestorm even before I started using it in your deck SG.)
Rain of Fire is very effective. Upgraded version costs only 5 and with 2 of those, you can kill everything on the table with less than 7 HP.
Other reason why it's so good in rainbow decks is Otyughs. With Rain of Fire you can soften enemies up and then eat them (or debuff enemy Otyughs). It also works pretty well with Eagle's Eye.
If it did do 4 damage, that would be just ridiculous (unless you up the cost). But if it did only 2, I would still probably take it.
Right now I'm playing rainbow with 2 Fire Storms.