I have asked myself this question many times. Some shroom enthusiasts, both from the Nordic countries and from elsewhere, think that Psilocybe semilanceata growing in Nordic regions are indeed more potent. As I was fed up with only asking this question and never answering it, I did some research. This is what I came up with.

Unknown origin:
0.98% psilocybin, 0.02% psilocin, 0.36% baeocystin (Gartz, 1994)
0.91-1.12% psilocybin, 0.15-0.21% baeocystin (Gartz)
1.1% psilocybin, <0.01% psilocin, 0.2% baeocystin (Anonymous, 1995)

Norway:
0.17-1.96% psilocybin (Christiansen, Rasmussen, Høiland, 1981)
0.5-2.5% psilocybin (Pedersen-Bjergaard, Sannes, Rasmussen, Tønnesen, 1997)

Finland:
0.62-2.37% psilocybin, 0.01-0.02% psilocin (Jokiranta, Mustola, Ohenoja, Airaksinen, 1984)

Now, with such a small number of tests, it can not be regarded as a fact that they are indeed stronger, it could very well be a statistical glitch. Also, note that the analysis from Norway and Finland showed not only higher amounts, but a much wider range.

Does anyone have any more analysis of Psilocybe semilanceata available? Or perhaps the full text to one or more of the cited references? The numbers are only taken from the summaries, as I do not want to pay for such articles.