I'll be adding a hack this spring. Fortunately I have already beefed up the forks, front brakes, and rear shocks. With the additional weight (my mates will insist that I become the official beer hauler) I thought if it was available that a rear brake upgrade may be prudent.
You need to remember that it is the tire that stop the bike. A fancier caliper is not going to make the rear tire grip more. If you have enough brake force to lock the tire up, that's as much as you need or can possibly use. I suggest putting your side hack on, loading it up with beer and seeing if you can lock up the rear wheel by standing on the brake. If you can, you have enough. If you can't, then start thinking about it. Just my opinion.