Hmm. I know from testing that opening the table takes longer than opening the form based on the query that uses the table. And, we're only talking about a table with ~1k records at this point. It's noticeably slower to open the table with subdatasheets enabled than it is without. I can't say I've noticed a difference on the forms.adding on ... my understanding is that just because you don't see them doesn't mean they aren't there! (by that, I mean being generated)
At any rate, they're only enabled as a developer's tool and I don't plan on relying on them long-term. I'll do some searching to see if there's a consensus on whether they have any "lasting effects".