Wheel question?

I have a totally stock 94 GT. I has the stock 16" wheels. I personally think they are ugly and i want to upgrade the looks. I was pondering on some wheels i found on the internet and i have a few questions. If anyone could help me out it would be great. The wheels i am pondering are the FR500 concept 18" wheels. The color is anthracite which is a darker grey color right? My stang is lazer red with a grey interior and i think the dark grey wheel would be a nice compliment. A friend who is a mechanic at Ford told me that when you upgrade in wheel size you loose performance is that correct? Also is it severe or minor. As i said my 94 Stang is stock and pretty slow and do not want to take any more performace away. Also if anyone has any pics of a sn-95 with the FR500 wheels on it that would be awesome.

Thanks a lot for you time
Jerry

Thanks for the ideas those wheels really do look sweet on that car. I was looking to get into some drag racing at the track i could throw on my 16's though.