I think I can help you out here, Beagle...
IIRC, when I cast these prior to shipping the mold, I went a little overboard toughening the alloy since the Lee WC I hp'd for myself had a tendency to blow all apart. So you'll have to play with your alloy a little to find that balance between not expanding to coming apart on impact.
Here's what they do in water jugs so far though.
Left to right...
Left--->As dropped, averaging 137gr. naked.
Middle--->Fired at 5yds in the 38sp. velocity range (about 800fps)
---Cleared 2 water jugs cleanly (not much reaction by the jugs) and stopped in the wet phonebook mess backdrop. Full weight retention, not much noticeable expansion.
Right--->Fired same distance in the medium .357 mag velocity range (for a PB lead WC bullet anyway...about 1050 fps or just a little shy of).
---Cleared 3 water jugs with some pretty good splash and stopped in the wet phonebook backstop. Expanded to .445 with full weight retention. One notable occurrence is that expansion is taking place as ballooning rather than mushrooming. It could likely maintain integrity through several more jumps in FPS before splitting and peeling occur from over-expansion. And even such, it would be frightening to see what kind of damage might occur once that happens. Interesting results so far.
All right Beagle, it's your baby--so now it's your turn to develop it fully!