I've never felt a noticeable different after plug change on any cars (tell a lie, did have a misfire at 4k that disappeared on one car), but then they've generally been done within the recommended time frame. However, whilst my C320 came with FSH, it was stamp only, not receipts, so I have no idea if the plugs have been done. The car feels fine and I can't feel any hesitation through the range, so I am loathe to spending £80+ on plugs, plus fitting, if there is nothing wrong with them. On my last car I just did it cos it was a simple 30 minute job and cost me £32 in plugs. So as the C is at 92000 with no record of plugs, is it worthwhile? Saying this outloud, I know the answer, but just wanted some 3rd party input.