mrs momo - which car seat do you currently have? I have travelled to 3 times back and forth to the US on Cathay econ with our giant Britax Marathon with no problems. (it's FAA approved as well) I think the Marathon was a bit larger than the seat specs, but it still fit. It was definitely snug and we did have a time unbuckling the plane's seatbelt.
Best to talk to Cathay as well to let them know you have baby car seat so they can book a window seat for the car seat.
The only problem we had was that our little one's legs can touch the seat in front and was kicking it. We fortunately had very understanding people sitting in front of us. (Can't imagine if they weren't and had to endure 14 hours of kicking...) If you get the bulkhead seats, you won't have problems, but you'll get a lot of bathroom or galley traffic.