Nah, its not like it is a huge deal. Getting mutated is not a problem. It is a matter of getting mutated in 15 or 20 seconds. Which is why, in the end, whichever route you go does not really matter. But switching to SoR will gain you a few seconds. Which is the exact purpose of this thread, SoI works bset on most (every?) fight, just some niche situations like this when it is actually worth swapping seals in terms of min maxxing, but not swapping won't make you wipe exactly.
I think this is the piece of the puzzle you are missing.
http://www.wowhead.com/spell=136050
Basically, it only procs from CS/HotR, SotR, Melee hits and SoR. Having SoR on will almost double your procs with it, making the adds die even faster. This is why SoR is so good there. And that is also why you do not miss out on any BH healing, since that proc doesnt proc BH and it can easily be 50% of your damage on the adds.
At 5 stacks, that is 100k damage every SoR proc, at 10 stacks it is 200k damage.
So it is not actually the SoR procs we are after, it is the Malformed Blood procs that SoR procs.
In the end, this is kinda extreme min-maxxing as the benefit is not huge, but if you are struggling with the enrage, you can easily gain a million or two more damage on the boss by swapping to SoR for killing the adds.