I'm with Ghono here. I forge for mastery on my guys simply for the reason that 80% of my damage comes from frost damage spells. On a percentage damage per point basis mastery gives a better improvement to these spells than haste.

The other thing I like about the mastery is that it effectively increases my burst. Sure over a full rune regen cycle this will even out but if I can open with 2-4 howling blasts from 30y which get an extra 30% from mastery, over a 1-3 second period, then the opening pressure is so much more than if those runes regenerate in 9 seconds instead of 10. If you are running in frost rather than unholy then you can argue that haste will be more value as it lowers your GCD too, but the extra run speed from unholy is too valuable to give up.

I think as boxers we have a different damage profile than single boxers which means that we tend to rely more on the easy ranged spells like howling blast for damage than the extra physical dps from whit hits and oblits. For this reason I believe that stat weighting should be more along the lines of:

+195 spell pen>5% hit>str>mastery>haste>crit

In arena spell pen is unbelievable. 95% percent of teams you face will be running 195 frost resist through auras. This gives something like 25% frost damage reduction so for those 195 stat points you effectively get a 25% increase of 80% of your damage. I completely underrated the need for this initially but to me, these numbers more than justify preferring this over hit even before cap. Where this falls down is if your enemy doesn't have any resist, the stat obviously has no value.

In 4.10 with the extra HBs coming from death runes and the buf to HB and FS, I think this is going to be even more true. What makes frost DKs so great for boxing is the flexibility of dps. Basically we are a resilient melee class that has awesome ranged spell based damage. Choose your stats to maximise this aspect rather than the melee physical damage and you will maximise this advantage.