There isn't really a max cap on crit benefit. Jut get more Haste/Crit and like Werst said it's largely interchangeable. I have seen a symbiotic relationship with Haste/Crit such that too much of one makes it less beneficial than the other, assuming you have 4pc.

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For single target, Mastery>Crit=Haste is not best. However, the more you AoE the better Mastery gets (and FAST).

As a trend, what I see as the number of targets, n, increases, then the rough estimate value of the stats is something like

Haste = 3+1.25n

Mastery = 2+1.5n

Crit = 3+n

(Those numbers aren't exact, but give a rough estimate of what I'm trying to get across if you think mathematically best)

That is, up until 5 targets Haste is very, very good. Past that Mastery scales up in value a ton. Crit is about equal to Haste, but falls behind the other stats with more Haste.

Single target Haste/Crit is very similar, with 2-4 cleave Haste is very, very good. Past that Mastery scales best.

HTH.