The question is, at what point is an alloy too soft to be driven at ~1800 fps when GC'ed and PC'ed?
In my experience, 25:1 is definitely at that point. It probably won't lead, but is likely to not shoot very accurately either at that speed. That depends a lot on the design as well though. It's also a lot softer than you need for good expansion, in my experience.
For what you described, at 1800-2000 fps with those bullets, straight clip-on WW alloy will give plenty of expansion. I would not go any softer than that for those speeds (such as mixing stick-on WW or pipe fittings with that alloy). If you think you've got an alloy that's considerably softer, like the pipe fitting & WW mix you described, you can buy a pound of antimony on Amazon and mix it into your lead alloy. 1 lb of antimony with 50 lb of soft lead will get you to about ~2%, which is just about right. You might want to reduce the amount of antimony you add, depending how much WW was in the mix; maybe 3/4 lb for that 50 lb of alloy.
FWIW mixing in antimony is not nearly as difficult as some claim. I start with ~15 lb of alloy in my 20 lb pot, add the antimony, and run it up to about 800° F. It takes a little while and lots of mixing, but does eventually mix in. For some reason, adding a lump of old bullet lube or candle wax helped it mix better than other fluxing methods I normally use; not sure the reason for that but there it is.
