Is phrase 他活说得好快 grammatical? I've been collecting memo sentences to distinguish between 活 and 话. The root  phrase is 他话说得好快 and I know it's grammatical, because it's given in a courpus.

What interests me is the grammaticality of another phrase, which we can get by changing (occasionally or intentionally) 话 into 活. 

So my question will be, do native speakers understand the phrase 他活说得好快 as a grammatical utterance and, if yes, what would it mean in English?
[活] and [话] are completely different word and cannot be used interchangeably. "他活说得好快" is not a correct sentence.

I think the reason you thought these 2 word can interchange is because of some common typos.