The OP is asking about Subject Tests, not SAT. I can’t speak for MIT. but I applied to colleges at the similar tier, and none had an issue with Subject Tests taken in 8th grade.
That said, the OP needs to be aware that the CB wipes out any scores pre-9th grade unless you specifically tell them not to.
As for the original question, the difference between a 790 and an 800 won’t be the difference between acceptance and rejection, but if he want to retry in a couple of years without a lot of prep, I see no harm.