Comeback: I'd probably let it pass without comment, myself, since you seem to agree with the statement. It just seems like it's not worth causing problems. But if you're certain your goal is to annoy people, you could come back with something like, "Really? You think so? Because he seems so easy to me. What were you doing wrong?"
2nd issue: I'm not sure I'd characterize that as "difficult." But yeah, it's common, at least around these parts. What you're talking about is emotional intensity... every emotion is just so much MORE with him than you're used to. That applies to anger, frustration, and sadness, but it also applies to empathy, love, and joy, too. The positive side of this is part of what makes these kids so endearing, so you take the good with the bad, and then work with them to help them channel those negative feelings in more appropriate ways.
My DD8 has exhibited that same kind of negativity, but she's getting better. There were times where I'd take her to the bounce house, we'd play for hours, and she'd come home and declare to her mom that she had a terrible time, because of the one time a kid pushed her. I'd have to remind her about all the good things that happened, and show her how all the good outweighed the little bad. Over time, she's become better able to sort that out for herself. She had a recent experience last weekend where she had ample reason to complain, but decided that it was overall a fun experience anyway. We've been working on this for about 4 years, though.
Your son's bossiness and negativity both point to perfectionism issues, another common problem for gifted kids. I find that the best way to deal with bossiness is to use the "How would you like it?" argument. You already indicated he can be very thoughtful, so here's a chance to help him tap that strength. We've also coached our DD and her playmates in the art of negotiation, and seen positive results. We don't solve their conflicts, we just mediate and give them ways to consider solving them themselves.
It could also be that the kids your DS is associating with are just a poor fit, and he needs a new group. If he was around older, more mature, and/or smarter kids, you might see a totally different set of behaviors from him.
