Okay, i know this thread is over a year old, but i just HAD to address something:
You guys are ALL idiots.
First of all, asking whether or not his eyes would get weaker by gaining a sharigan is just dumb.
If you add one good thing onto another good thing do you get something worse? NO, you combine the affects of both to get something even better. So assuming that naruto has better eyes than sharigan, if you were to give him the sharigan his eyes would do one of two things:
1. Gain all of the effects of the sharigan but his perception not be affected because it's already better or
2. Gain all of the effects of the sharigan AND have the visual perception of the sharigan added on to the perception he already has.
Here's an example... If you are a buff guy with a large amount of muscle mass, and you COMBINE with a scrawny guy with a little amount of muscle mass, You wouldn't LOSE any muscle mass, you'd just ADD the scrawny guys muscle mass unto your own.
There's so many people in this thread thinking themselves "smart", and calling others "dumb", yet NONE of you could solve this very simple problem. Maybe stop trying to prove others wrong and just use your brains for once.