Howdy,
In answer to your first question, modern saws generally cut faster than older ones. If you get your leg in there because you pushed a dull chain on through a cut, they will be found to cut quite nicely. A lot better in the leg than in the wood.
As far as chainbrakes themselves, yes they can help in certain circumstances, such as kickback straight up over the top, or if they are inertia brakes, even at off angles. A couple of my friends have managed to miss getting sliced because these things actually did work. The handguard is almost as important as the brake however, but you don't hear much about it. It seems the vast majority of chainsaw operators, yes even the pros, can not help but get their thumb up there on top of that front handle bar, and when they do, it is only a quesion of time before the hand slips off and into the top chain. I got to patch a guy up from this once, and I assure you, he will never again run a saw without a chainbrake and hand guard. (He was limbing a standing jackpine).
Myself; the only scar on me from a chainsaw is from a cutter slice when the file slipped due to a poor quality chain having chrome all over the place so the file could not cut, and only slid.
No safety device can always protect a loose nut behind the handlebars from himself, when he insists on using a machine that is designed to cut.
Regards,
Walt Galer