I feel like posting today so...
1 - Tiffen filters are okay, but they are plain glass filters, without coating. You will increase your chance of flare with these filters. This will, of course, lead to decreased contrast and, if really bad, those little flare marks across your image. My Nikon 18-35 is already flare-prone by itself, and I have to use coated filters on it, even with a hood.
2 - I have not noticed any kind of decrease in sharpness from Tiffen filters. so other than the flare leading to reduced contrast, I don't think you'd notice the difference. Of course, if you get flare, that's a pretty big issue.
3 - I'm not sure you'll do any worse with a cheaper filter.
4 - I use Hoya multicoated filters. Many recommend B+W. They're multicoated and are equal in that regard, but the B+W ones won't bind and get stuck on your lens as often. I just couldn't afford the price difference. Multicoated filters are expensive.
5 - Your other option would be to go with a rectangular filter system like a Cokin or Lee system. I am moving towards this way for much of my filters, but it's not as convenient and it's a lot more cumbersome than screw-in filters.