So, I filed down my front sight as instructed, and- sure enough -it was way above the boresight. This had me totally confused, so I did a little research. It turns out (you probably already know this) that the pistol is in recoil before the bullet leaves the barrel, bringing the the Point of Departure (what the boresighter was telling me) way up. The lighter your load/bullet, the longer the bullet spends in the barrel, and the higher your point of impact is going to be. That's why you need all that extra front sight, for those real powder-puff loads.
Well, as I said, this is probably old news to you all. Thought I'd post my finding in case anyone as dense as me is googling for this.