Macs are a lot of sound a fury but I doubt you will see any real power gain. I happen to like the looks of them on other bikes (haven't seen a set for the xs400). The chrome on them is usually shit (will begin to rust within a year) so if you can buy a black one and then repaint it with some high quality high temp paint. As far as the price I think they are worth it because 1) they are the only game in town for xs400 exhaust, and 2) they are not expensive pipes and they really have a nice sound.
hey, we have a '78 xs400...and it had the mac 2-1 on it, black... wasn't in very good shape though it was rusted...We're going to sand it down and get it powder coated, hopefully it stands up better, so I would have to agree with geeto on the painting...I can only imagine if it were chrome. but, it does sound mean...I couldn't believe I was hearing a lil' 400.... as for performance...everybody I've talked to says to leave the 2-1, I'm assuming it has something to do with a more constant back pressure, but I'm not sure. (if anybody knows...I'd love to hear) I tend to like the look of 2-2 pipes on each side, but it doesn't look bad either....
I bought one new, back in like 99 or something for my old kz400. I think it didnt fit all that well, but it was because of the baked on ceramic coating it had. Eventually I got it on and it sounded good for a 400, throaty but not tinny. I think they looked good too. I'll post up a pic soon on mine if I can find one.
Got the pipes in the mail today, and already got them on. They sound really good but I have not really had a chance to ride it very far. No noticable power gain yet. I'll post a pic when the k&n's and new handlebars come in. They look a thousand times better than the 27yr old pipes that were on it.