That's a pretty nice frame to begin with, from what I've read Tange 2 is nice stuff (someone correct me if I'm wrong) but I doubt a Kilo or IRO frame would be much of if any of an upgrade. Saddle is important, is it comfortable? If so then leave it alone, if not, maybe try some out at a shop or look for some that are popular. People around here love Brooks saddles but I've never ridden one. If you looking for racier looking saddles nashbar has a nice knockoff of a Fizik Airone (Glide G2), people also seem to like it.

The biggest and best upgrade you can make to that bike is a front brake. But you're going to need a long reach brake I think Tektro makes a long reach brake that would work, but you might have to drill the back of your fork out to fit the recessed nut.

Also, ditch the purple chain and pick something up a little more stretch resistant. I ride a KMC z510 and it's seemed to hold up pretty well.

That is a very nice bike, no need to "upgrade" to another bike. Also, fenders may look nice.