>chitin appears to be primarily responsible for shroom's stomach ache
>mushrooms in general are 8-45% chitin
>shrimp are 15% shell
>shrimp shells are 20-30% chitin
>.45/(.15*.20)= 15 and .8/(.15*.30) = ~1.8
So in theory eating 0.8 to 14 times as much regular food as your dose of shrooms should only give you only as bad of a stomach ache as an equivalent amount meal consisting only of shelled shrimp. (using the probably bullshit data I found online and assuming psilocybin has a negligible effect)
It would make sense with my past experiences with shrooms.