Mine is 3" SI/ 2.5" DO. I wouldnt bother reducing it to 2.25". Seems like more work to me. Also, if the price of getting it welded is too much, check out bandclamps. I love those things and they never leak.
Not extra work for me I've read online (and yes I know it could change depending on the shop) that most shops charge around $60 for a complete exhaust system. I think mandrel bends would be a little more expensive though. And thanks for the link, I'll keep them in mind.