When stressed in a bad way (frustration, not getting things done, being dumped on, dealing with situations I can't control, etc) I can get very touchy and emotional (basically a 180 of my usual self). I will withdraw, "I don't need your help, please go away and let me think" is what I'm thinking if not saying. I can drop the ball BAD on things that are "peripheral" to me (but shouldn't be) like prior social engagements, paying bills, etc. People get pissed off, for good reason.
I also agree that moderate stress can force me to focus/be more productive, but past a certain point (uncontrollable) it probably makes me less efficient on top of things.
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I will also add:
I have a bad tendency to take on more than I should, and to accommodate more than I should. Some part of me knows this, so when extreme stress hits, these things get dropped like a hot rock. Suddenly I'm not willing to bend/flex when I would before, I'm not willing to do things I did without complaint before. This can surprise/hurt people.