I wasn't trying to give a real number, I was saying that 'too hot, planetwide' is rather unrealistic. The initial ass-number was '6', why does your argument rely on numbers an order of magnitude larger?
I think it's perfectly reasonable to postpone worrying about 50+ degrees until it's already shifted at least 5. If it turns out 5 is terrible, we'll have hundreds of years to implement plans to stop 50.
If you think 5 degrees is catastrophic, then sure worry about that in 2013. But it won't make the planet uninhabitable.
The point is: there is no predictable upper limit, period.