.46lbs if you want to be exact. They aren't heavy.
The Flight is .28
If you're having jaw fatigue with the XTX you're doing something wrong, especially if using the Comfobites. You shouldn't be clenching or biting or anything, your jaw should be completely relaxed with that mouthpiece, the lip holds the weight by your upper teeth and your lower jaw is just kind of hanging out. If you're having that much trouble with the comfobite, try a Seacure, but all that does is put the bite tabs all the way down your molars and doesn't really help out all that much compared to the Comfobite. This is an Apeks forum, so Kyle is going to yell at me for it, but I don't particularly care right now. There is a knockoff of the Comfobite that is sold by various companies, but Dive Gear Express is my preferred place for it. This is a blatant knockoff of the actual Aqualung Comfobite that is a brilliant mouthpiece, but it is a bit softer and not quite as thick as the original so it may fit your mouth better.