1) The windscreen is nice to have when you get to high speeds, keeps the wind from pushing you around. That's its only real purpose. If you like the bike without the windscreen, that's cool. Personally, I like to have mine. A dark smoke one would look good on that bike - IMHO.

2) Lowering the bike does change the geometry. You are going to find that the bike is more stable at higher straight-line speeds, but the bike won't lean as far without scraping. You'll probably find that you need to go a little slower through corners. I say probably because I have no idea how fast you could corner before lowering the bike. If you don't ride in the mtns or on the track, it probably won't hurt you at all to have lowered it. Should help a little bit at the dragstrip.

3) I never liked the stock mirrors on the Katana. But I don't like to ride in Atlanta traffic without mirrors.