Both the 416 Rem and the 400 H&H, are basically the same design, a 375 necked up..either is an excellent round and neither need to be improved as those torpedeo shaped cases will extract even when the case malfunctions, they don't stick in the chamber like an improved case would do...and improving gains nothing anyway in most cases...

I think the 416 Rem is by far the more practical and factory ammo is available, but the 400 is an interresting round, would be fun to own one...I will probably build one on a Mauser one day..