Should medical conditions that affected performance be stated in personal/research statement? And how?
Your potential advisor is interested in (mostly) one thing: your potential as a graduate student. To that end, your personal statement/interviews/etc. should serve to convince him that you will be able to do excellent research. Any issues that could be a potential problem should be noted and addressed.

With that background, do you think this condition could be perceived as affecting your output as a graduate student? From the way you posed it, I would suggest that the answer is "yes". As such, I would mention the condition towards the end of the letter/interview, but I would state  with confidence that past performance has demonstrated that it will not affect output, and you're confident it would not be a problem in the future. Don't dwell; just mention it as you would a disclosure and move on.