Edit: just to preface the below, remember that WrathCalcs isn't 100% accurate and there's still an issue (though it may not be) with DoC doing less DPS for multi-targets, but here's some graphs to illustrate reforging in and out of crit for mastery.
From these graphs, you can see that the curve stays relatively the same shape regardless of how many targets you're facing. Additionally, each curve features a hump or equilibrium point at around 23k crit, 2k mastery. You can also see having 0 crit is much, much worse than having 0 mastery, and we'd expect that since it completely cuts out part of our class (SS procs). Just food for thought, and I'm not making any extreme claims, but crit seems to be the better stat even with DoC enabled.
Hamlet touched slightly on why DoC may be showing less DPS with more targets over on EJ: http://elitistjerks.com/f73/t130885-...4/#post2329703. WrathCalcs could be completely correct, but I'm uncertain on the topic for now. I think supplementary evidence from SimCraft would be good in this regard.