Actually, increased firepower doesn't help much. What ensures party survivability in CoC is intelligent use of resources available. No amount of firepower should be able to help them out in the climactic conflict, their intelligence should be their strongest asset. Even magic is a bad idea.
In that specific adventure though, what "intelligent use of resources" would've solved the problem? And it looks like increased firepower would've solved the problem, intelligent or not.
I'm not recommending that; I'm asking how it wouldn't be the case.