I've seen at least a couple 'mastery stat is best' posts on various forums assuming the mastery stat on gear would affect all 3. How good mastery will be is hard to say. A lot will depend on how much the ilevel budget for mastery is. The few posts on the matter I've seen so far again assume that the budget will be the same as for str, but this isn't necessarily true. We might see 40Str gems and say 25mastery gems of the same quality. Still plenty of ways for blizzard to tweak the stat so it doesn't trump all the other stats we should be desiring. With blizzard having stated they don't like the 'one stat to rule them all', it would be a design mistake to now introduce a new stat that does just that.
Gems with "rating" stats match across the board. For example: 10 str/hit/crit/haste/etc are the same quality gems. Only spellpower and AP "break" this rule (with AP at double Str or Agi levels, which makes perfect sense mechanics-wise). There is no reason to break this convention.
If Mastery is considered "too powerful" they can tweak it at two other places - rating to Mastery value/% or in the talent trees themselves.
It takes 45 crit rating for 1% crit, but only 32 hit rating for 1% hit. 1 Crit Rating and 1 Hit Rating have the same "cost" on gear. Mastery can easily have some other conversion factor. Mastery too good for everyone? Require more rating per 1%. Not good enough? Reduce the rating.
If this does not fully balance every class/spec then they'll just tweak the 1% Mastery to Talent Tree bonus conversion. Say conversion is 1% Mastery for .25% Holy Damage increase (arbitrary number). Ret's too weak? Make 1% Mastery .27% Holy Damage. Too Strong? Make 1% only .22% Holy Damage.
They have 2 very good knobs to turn for very detailed changes, why would they alter gems?
Any posts you have seen have been working off of the numbers currently active in Beta testing. Which are, as we all know, subject to change. Unless/until they do change, they are valid comparisons.