If your SBs are 2.03s then you have approx 19% haste making your GCD ~1.2sec. This means that you are spending 2.4 sec to cast 2 Fel Flames. Fel Flame costs 5% of your base mana and SB costs 5.5%. Going by the numbers you give above you are gaining 5.8% dps (between the two not overall) by using Fel Flame. However you are burning somewhere in the realm of 55% more mana to do it. The extra lifetaps from this will more than offset this.