I cant say how the magazine effected trigger pull but those Promags aren't worth the plastic bag they come in.. I bought three for my little Bersa Thunder, all three were bad, springs so weak the ammo fell out of the mags... I've never seen anything like it, you'd think Promag was owned by Taurus or something...
(Yes, not so cheap shot at both because both hit me in the wallet in painful ways.)
Still that P220 should be as smooth as silk... My pull in DA feels to be around 12.5 lbs. or so and something around 3 or 4 in SA.
I'm hoping to buy a second one in 2014. 2013 has become by default the year of the Sig 716... (pending)
But in a wild guess to your question, I would suggest that the Promag is not in spec and is pushing on something causing the gun to have this issue.