Its a NW T. Venom affects people differently, I wouldn't take their word for it unless they've experienced a bite from it themselves and I'm pretty sure they haven't been bitten yet to make definitive claims.The bite report section of the forum is really your best bet. (You probably already read it but here's a link) [Linky] I'm sure first hand accounts of people being bitten would give you a better insight as to what you're trying to find out. First hand experience is way better than assertions and assumptions. Also check out the bite reports for P.cambridgei, P.pulcher, & P.reduncus as venom potency would be likely similar.
BTW P.irminia aren't really aggressive from my experience, but they are quite protective of their homes/hides. If you don't want threat displays from a P.irminia, I recommend to not place the mouth/opening of their hide in front of where you open your enclosure. Otherwise, you will most often be greeted by an angry tarantula.