Fitment seems to depend on each set, the ones I had were fine but I have heard mixed reports. You will certainly need to swap them out come MOT time, unless you have a very friendly tester.
The sound is noticeably affected, they sound AMAZING. They are quite a bit louder, but only really when you put the foot in. I noticed my MPG went down, but that was probably more due to driving everywhere with a massive grin on my face because of the noise I wouldn't say it was much difference though and with what the Z gets anyway it wasn't significant to me.