Lay offs are never a nice stress free process, any system you use to determine who to let go will suck badly for everyone involved.
I couldn't ever overlook the gross misunderstanding of statistics here. The "bell curve" is an asymptotic distribution law, and even assuming that the distribution of performance is indeed normal, it is outright idiotic to fit the bell curve with as low as 10 people. The chance that no one out of 10 random people will be really at the lowest 5% of performance, is as high as 60%. The HR even uses an Excel sheet to even draw a normal distribution...
I have seen many good people's careers stopped in my farm (and it is a big farm), because of this "bell curve".
Layoffs are about speed, not accuracy. The faster they're done, the less damage they do. The only thing that matters besides the number of people to lay off is not losing your best people, which comes down to speed (the longer the mess lasts, the more likely your best people will leave) and being accurate enough to not accidentally lay off your best people.
Seems to me that stack racking, however awful it is ethically, emotionally and statistically, at least covers that well enough.