One adavantage the 458 SOCOM is that you can use the 50AE spent cassing and size it to work on the 458 SOCOM.
.458 socom brass is longer and it headspaces on the case mouth so you wouldn't be able to use .50ae brass for it.
Over the last 1 1/2 years I've asked Alex repeatedly to make a conversion kit in 450 Bushmaster and/or 458 SOCOM for my XCR-L his answer is the bolt would not be strong enough and would be prone to breaking so not happening.
Sounds a little like he is feeding you a line. .458 socom is lower pressure (than .223Rem) and uses a modified (weakened) ar bolt, so one would think the xcr should be able to handle it.