Actually non-ECC it's about 2% faster at the same clock. Here is a nice article that covers pros and cons.Can you have non-ECC entirely? Yes, it's like putting 87 octane into an engine that takes 93 octane. It'll run, but not to full performance (in this case the error correcting code feature won't work)
Pretty much. Nothing is ever free. You want error correction you pay a little in ultimate performance. A couple % won't be possible to tell the difference in day to day use so its probably not worth worrying about. If you are doing weeks long calculations that can all be wasted if a bit is flipped you can bet it's worth way more than the little performance hit.