When is a kayak overloaded?

Skip the carry toggles.
I’d advise against carrying a loaded kayak by the carry handles. Lift it by the hull then shoulder it about 1/4 of the way in at each end like a log. This centers the weight of the gear in the ends over each portager’s shoulder. If you can’t pick a boat up over your head, you probably shouldn’t be carrying it loaded any which way.

Pitch-poling, capsizing, broaching, etc. will be harder to do (read less likely) if you load your boat logistically as others have said. The ride can also be smoother but wetter as you’ll tend to ride right through smaller waves rather than bobbing ontop of them.

I remember paddling a higher volume (for me), heavily laiden, poly boat in steep waves one trip where I could actually feel the floor flex as the middle section wanted to surface while the ends didn’t. Thankfully, they don’t make many sea kayaks like that anymore (single layer HDPE).


"when it handles poorly"
That really is the best answer to your original question.

There are so many variables in design that going by published numbers may be of little help.