It doesn't undermine your point, but if you mean gross yearly wage $25k is not an amazing salary for senior software developers in Poland (I guess it depends where in Poland).
I find Poles to be among the very best, but it may be a bias in the sample. Most of the ones I know were motivated enough to move to the States for awesome jobs.
I worked with and known a bunch of Polish devs (in Poland), certainly a very high quality talent pool. There are also great devs in Romania and a lot of the countries that were part of former-Yugoslavia.
I never understood how there can be a significant wage gap between these countries and Western Europe for jobs that are anyway done online.
What about countries? In my Poland $25k would be an amazing salary for a senior while in USA fresh grads can earn $80k. Are they more productive?
... at the same time, given same seniority, job and location - I'd be willing to say it wouldn't be a bad heuristic.