Rebuild or leave trans?

Now going back to the strength of the trans. I could take the time to rebuild it, I'm sure it's not too hard just time consuming. In completely stock form with the addition of a shift kit would it live happily behind 400 ish hp? Majority street driven. Does the shift kit add strength?

Now I assume if I rebuild it with some better parts it would handle that power with ease, but I am not reaaly wanting to tear into if I don't have to. From what I understand shift kits aren't terribly expensive anyways.