Stamets talks about this in the GGMM too:

"The most notable carcinogenic hydrazine from this mushrooms is agaritine, a powerfull mutagen which is activated by the mushroom enzyme tyrosinase making it heat stable"

"Walton et al. 1998 asserted, however, that the mutagenic and premutagenic compounds are not affected by quick cooking (10 minutes at437f) but are only slightly reduced by prolonged heat treatment in boiling water for 4 hours at 212 f. A study of blanched canned mushrooms showed that the agaritine content was reduced tenfold in comparison to fresh mushrooms... However this reduction may have been due to the leaching of hydrazines into the surrounding water used for blanching, in combination with prolonged high pressure steaming process used for caning."

"Hashida et al 1990) Hashidas study reported marked reduction of agaritines from boiling water at 212 f for 10 minutes, a report in direct contradiction to Waltons 1998 study'"

In my opinion since this is the most popular mushroom sold commercially, bringing in big money for the producers, any ill effects are kept hidden as much as possible. I beleive that the contradicting results arrise from this "hiding" there is just too much at stake for the producers, if the general public were to become aware of the possible dangers. Especially the consumption of raw portabellos. They should really try and select a strain with extra low carcinogenic properties.