My Great Auk has climbed in weight to 75 lbs. (newbie epoxy mistakes, etc.... recessed carbon rim overbuilt.... )
For me the main issue with weight is carrying it, and loading on my car racks. Once in the water the weight doesn't seem to be much of an issue.
I have a plastic beater yak that is much lighter, but being much shorter isn't as fast as the longer heavier G Auk. I'm sure lighter is better, but on the water having or not having an extra pound or two on the stems probably doesn't matter much--except perhaps in competition.

Adding the weight of cameras, etc.... also doesn't seem to be much of an issue unless they have to be carried or portaged with the kayak at the same time. (the bicycle rule is (or was) an ounce on the wheel is worth a pound on the frame. Perhaps for kayaks it might be 'ten pounds on my shoulder feels like one pound in the water.'

I'm building a Mystery which is much narrower, so likely to be less stable. I'm wondering if there are any numbers (graphs) on the added stability of adding long narrow weights just inside the keel?

I doubt anyone would want to glue down a lot of weight, but removable weight? Would 20 pounds add some stability? which could be carried in the car as an option on days the sea is rough. . . . .
(The CG for a seated paddler though is probably the last vertebrae, and this can't be more than 4" from the bottom of the hull. At 180 lbs .....
I think I just disproved my idea....
Now that I'm rambling..... who's going to be the first to hack a Segway gyro and install it in a kayak? Nick, his brother, Tom Yost?