There are 3.5L V8 X350's kicking around, but not R models.
I saw a couple advertised, black ones, that looked like XJR's, with Sepang wheels, and the blacked out grille & rear trim.
But they aren't R's, maybe just Sport's.
According to a book I have called "Jaguar All the Cars" there were 5 specs of the 3.5
The 3.5 Sport SWB was dropped in 2006 (only 523 were ever produced from 2003-2006) and the rest were all made up to and including 2008