I'd avoid the lead in the USP or Glock, unless you do some real science experiments. I know what they say about lead buildup in polygonally-rifled barrels, I think.
-I also know a Master level IDPA shooter near me who's been shooting cast bullets in his Glocks for a long time, without problems. I don't think he shoots more than 200 or so at a time without de-leading the barrel somehow, though.
-I also know the old director of action shooting at my club says he saw a guy blow a Glock up right in front of him by shooting lead bullets in it.
Maybe both guys are right, but even if they are, the second guy saw a Glock blow up from lead bullets. My intuition tells me there are probably a wide range of circumstances under which you can shoot lead in a poly barrel without excessive leading, but change something (hardness of alloy, size of bullet relative to bore size, etc) and maybe you can lead up quickly and kaboom.
Or, another cool thing you could do is to use some sort of case filler such as cream of wheat to prevent and to remove leading. I used to do that before I got my electric "Lead Out" system. It adds one or two steps to the loading process, but in my experience it works. Work up loads very carefully especially with fast pistol powders. Use something like that, and you should be able to shoot all the lead you want in a poly barrel without leading. Just need to somehow do controlled experiments to make sure it works as described.
[LATER] - T must have posted the BrianEnos forum link while I was typing. The long and short of it looks to me that you may be able to safely shoot lead in a poly barrel, and that you can easily screw something up and blow up the universe trying, too. Don't do it if you don't know what you're doing.