Nice report but at -11*C (12*F) you won't catch me out there shooting. I'm getting too old for that nonsense! I'll wait until its a bit warmer out or else go down to LA and visit our dear friend Semisane. I believe that would be an interesting meeting. We can shoot a bunch of BP and probably a lot of BS too!
I know you said the BH209 in the GPH was "just to see" and I agree. I don't see it very practical to be using a duplex load in the field. And I don't think a critter is going to care either way what propellant you used. Additionally, your accuracy wasn't any better using the duplex over the straight BP. I think your GPH would probably shoot better with a heavier/longer bullet anyway with its 1:32 twist.