Is it possible to neck down a .577 Tyrannosaur ocr .600 OK to caliber .30?
What velocitys would be possible? The .30-378 Wby take around 120 grains of powder. But the .577 T-rex take near 200 grains!

Overkill, it's possible, like anything else. You could neck down the BMG to 30 cal.
Problem is, with even the slowest powders, you don't really gain anything but blast and recoil.
Look at the AR reloading pages, and some of the big-cased 30's there. Especially when you're talking sporter-length barrels, seems you can't really use much more room than the Jeffery gives.
Now if you're just rich and bored, you might try cutting it down to 2.2" and putting it in a short action, see what it'll do.