I know the 2004 models were substantially improved and have a much better reputation than the 1997-1990 model XJ8s. But are the 2003 still known to suffer from timing chain tensioner issues and frequent transmission failures as well as the Nikasil issues? Are the 2003 models truly any more reliable than the early model 4.0 liters?

 

Quick answers. By 2002, Primary chains were Morse style, tensioners were metal, Nikasil was gone, and I am not sure about transmission input hub, but mine has 160,000 miles on the original transmission. And yes, I drive the P**S out of it.
So, yes 2002- 2003 are the best of the x308 series.