I cant comment on performance changes on each exhaust, someone on here may have had one fitted and have some RR graphs showing any change though.
Depends on whether you get a sports cat (still a cat but with a lower number of cells therefore less restrictive) or a de-cat which is a pipe without a cat at all, they can be either unsilenced (just a straight pipe) or silenced (with a silencer in place of the cat) The cost will vary depending on which, from about £100 for an unsilenced decat, to anything upto about £600 for a sports cat.