a) Does it? Yes. None of this stuff will bear scrutiny when slowed down to super slowmo (at least, not without cover):
Strike vanish, Snap change, flipstick, Shapeshifter change, silk through mike stand (and other silk effects sharing that method), and arguably the common coin gaff named after a dolphin. Also that shakey thing some people do when one billiard ball becomes two.
b) Are they really necessary? Not really since there are alternatives to all of them - but the point of all of these is to do nicely visual things with little or no cover. Best avoided on film but extremely good in person. Horses for courses.
c) Experience? I'm a fan of both flipstick and that coin gaff so yeah, I'd say it's true, for those examples. But - slower is generally better and even these things (I think) all look better when the surrounding stuff is done at a relaxed, sedate pace. Especially with flipstick, there are several other (slower, bigger) movements going on that deceive the eye (either toss up and away from you, or break over the knee).
It's not that these things are literally faster than the eye is capable of "seeing", it's that they're fast enough to escape perception when accompanied by other distracting factors. On the general speed in performance point, I definitely agree that slower is better. I think that quite a lot of the time audiences are just nodding and smiling, trying to look like they're keeping up and hoping they don't have to answer any questions later. Certainly that's what I'm thinking most of the time when I'm in the audience.

So I'm in the "Yes" camp. The quickness of the hand does deceive the eye.