One of the better sources for these differences and their magnitude is
Peopleware. Instead of citing vague research "somewhere", they did their own primary research. More specifically they set up coding wars where different programmers at different companies assigned to the same task independently.
They found 10x differences in time spent on the average coding war (which was typically a pretty small sample) with those finishing faster typically producing programs that worked better. So by any reasonable measure, at least 10x productivity.
There is your 10x, right? Wrong. They found that the best predictor of programmer performance was the performance of another programmer at the same company. Furthermore they managed to correlate a lot of that performance factor to specific factors, such as having a phone that turned off, a private room, adequate desk space, and so on. There was still something like an unexplained factor of 3 left over, but they didn't know whether it was environment variables they had not looked at (eg training), individual ability (which might be correlated across institutions), etc.
It is true that this research happened in the 80s, before the Internet was widespread. I would love to see it replicated again.
(Note, I'm aware of the existence of many other poorly controlled studies, see http://www.flownet.com/gat/papers/lisp-java.pdf for an example, but what is critical in the Peopleware study is that they acquired information both about productivity and about factors that might cause productivity differences.)