I think you've named the only two real weak points of the car! I've heard rumours of superchargers needing replacing at 150k, but nothing concrete. The X308 seems to be a much less troublesome and far more thoroughly engineered car than the S type all round - a well maintained one with sorted timing tensioners is unlikely to give you any serious trouble. The gearbox in the XJR gives less trouble than the standard XJ8 too, although it's still the same "sealed for life" rubbish. If you're looking at anything over 60k I'd want to find out if the gearbox oil has been changed, as actually believing the sealed for life rubbish seems to lead to a life of 100 - 120k ish.
Same as all XJs though, check for suspension wear. I've heard that the discs/pads for the optional Brembo brakes are hilariously expensive, so I'd be certain you find the standard brakes inadequate before buying a car with the Brembo setup.