Well... assuming four toons have the firepower to out-DPS one healer...
Which I'm not sure if that is the case, but once it is...

Stacking a combination of Resilience and/or Stamina, to get a longer absolute survival period, has been a good strategy for boxers in the past.
In both WotLK and Cataclsym, my DK's favorite gem was 40 Res.
We have the luxury of not needing the absolute most possible killing power on our only toon, as long as our combine DPS gets the job done.

More competitive levels of arena (for me 1600+), is about the only place where you need every choice to be optimal or near optimal to succeed.
In random battlegrounds, a 5-boxer with a half-decent composition will effectively be a raid boss.
I'd imagine 10-boxing will be the same...

Still, while boxing gives immense kill power, it doesn't necessarily let you achieve the victory conditions for the battleground.