Not when you have a selection bias, it isn't. If your sample selection is consistently biased, no sample size will be large enough.
Agreed, but outside math class you have to look at the percentage and make an educated guess about how special they realistically could be. If you have a thousand employees your number one is probably a genius and your very worst a moron. The 10th from the top is also probably pretty smart and 10th from the bottom pretty stupid. The 50th smartest isn't aren't all that special though, if he can be more efficient with a Mac well it seems worth trying the top 100 or 200 too. It could of course theoretically be that performance drops off a cliff because it takes some minimum skill and understanding you just dipped below, but realistically if that happens it's probably the kind of thing only 1% or 0.1% of your employees grok. If 5% can use a Mac so can probably most of them.