H&K really does make the best mags for their own firearms. Im not aware of after market steel mags for the full size USP .40sw. I could be wrong tho, the after market is hard to keep up with, but they do come up with some nice stuff from time to time.
Im staying with the factory polymer high-caps and white 16 rounders (.40sw) for the Jet Funnel Kit...when its installed.