Disk's are about more than just stopping power, and rear disk's even more so. Disk's bring the benefit of less fade and make it much easier to modulate braking when traction is limited.
For street driving, I think the biggest disadvantage to drums is the issue of self energizing drums locking up when traction is limited. Lot's of traction? Probably stop well enough - at least once. Not much traction? Better hope the rear drums don't lock up and introduce a dangerous ballet move.
