I fly in all kinds of airlines (although never Cathay) and they all have the 15 pounds limit, but they never check it. I wouldn't worry. If you are still worried, just keep the back pack on your back or on the floor close to your feet in the counter and outside their view. I bet they won't check the weight. They will only make you check stuff in if it doesn't fit in the cabin.
Luiz
Luiz,
I fly Cathay all the time, and have been doing so for more than 20 years. CP used to be a great airline, but they're not any more. You're right that Cathay is usually reasonable about carry-on bags (but the airport authorities in HK aren't - see below!). CP is very strict on the checked bag weight allowance. If you're more than a few pounds over, you will pay. (I had to pay over HK$1200 (about US$160) when I flew back from HK to Bali two weeks ago.)
More important, if you're routing through Hong Kong (i.e., leaving the airport and rechecking in - not just transiting between flights), the airport people will not allow you to carry more than one carry-on unless you have a business or first class boarding pass or a special tag issued by Cathay, which Cathay usually will not give you. HK Int'l is the only airport in the region that does this, and it's infuriating. (You're probably ok if you're just transiting.) The only way I've been able to finess this is to split up cameras and lenses in both carry-ons (my Lowepro back pack and a second carry-on, a Pelican case that just fits the dimensions limit rack at the airport). If you show the people at the entrance to the immigration area that you have cameras/lenses in both carry-ons, and provided the bags aren't too big, they might give you a pass, but there's no guarantee. (That's why my second bag is a Pelican, which can be checked if necessary. But keep in mind that if the airport people insist that the second carry-on be checked, Cathay can and usually will charge you for the additional checked piece and its weight.)
One more thing, if your Cathay ticket is part of an multiple international ticket which includes an origin in the US (and maybe Europe, I'm not sure), then they are supposed to give you a 20 kg weight allowance, not 15, and that usually means you can get by with 25 kg in practice. The 15 kg limit only applies to tickets covering journeys with one or more legs entirely within the Asian region.
I hate flying, hate airlines. But if I have the choice, I much prefer to deal with problems like these on Singapore Air than CP.
Frogfish