I wouldn't deluge the adcoms with extra stuff. I would only provide an extra letter if it has DRAMATICALLY DIFFERENT material (eg: you've never done research, but since applying, you have done dedicated research and will be doing a presentation and/or publishing. MAYBE you could justify a letter from your PI.) Most of my schools only have a committee letter from me, and I've had >10 II. Another letter with roughly the same material will not help you. Only update schools with stuff if it is SUBSTANTIAL, not just because you haven't heard from them in a few months.
Basically, I would only suggest an additional LOR if it is a different professor who is providing a much different perspective that is not indicated elsewhere in your app.
GOOD LUCK!