Anecdotally I think CX will generally be quite lenient to J pax assuming you have less than 10kg over the limit. But I don't know if there is a hard and fast rule.
Well on my last 2 J trips on CX I did have a problem - leaving the USA they were fine both times however it's the HKG check-in both times that nailed me and they were not lenient at all. Matter of fact on the last trip I ended up throwing away a pair of shoes to lighten the load and I thought the CX Agent was going to pass out when I did this (the shoes were old) Part of my problem is my luggage already without anything in it tends to be heavy on it's own so does not take much to put it over their limit.