Attempting to control the consumer market by choosing a lesser chip does sound like something IBM would do. They were all about protecting their turf (as400 and mainframes).
That's a good markup, and it doesn't seem to do anything that a regular rack mounted intel box couldn't do. It probably has some reserve in terms of IO and reliability but for the spot where it's sitting it is total overkill (ancient ERP system only running on that hardware).
I recommend Frank Soltis' books about it, although they are very expensive. For example an earlier edition is http://www.amazon.com/Inside-As-400-Frank-Soltis/dp/18824191...
As an example there is extreme backwards compatibility, pervasive database, single address space (a different meaning of virtual memory than you are used to), sharing, tagged and protected pointers, clustering etc.