Did you see anything published from Speer saying what the minimum velocity is for expansion of the Deep Curls??
I haven't seen anything like that published by Speer
HEAD. But I think it's fairly safe to make some assumptions.
The 250 grain DC is designed top expand at .45 Colt handgun velocities, where the normal muzzle velocity is 700/800 fps and even a very hot load is under 1000 fps. So I would expect it to expand at very low velocities, probably down as low as 500 fps. However, we have good evidence that it won't fragment and holds together beautifully at whatever velocity you can push it.
The 300 grain DC is designed for the .454 Casull handgun round, which claims a muzzle velocity of 1600 fps. So I would expect expansion down to 800/900 fps (probably lower). It too will hold together as fast as you can push it.