the V8 XJR and the AM have basically nothing in common. The AM was based on a heavily modified XJS platform that in the end was basically not an XJS platform.
To the original poster: as in any new model series the 1st few years are going to be a pain in the butt, but man - the XJR is such a pimp car, fast and GOOD looking I dont think it could stop me if I had the money!! XJR is one of my dream cars! (think Arden . . .)