Sorry to contradict, but all X150 (aluminium) models, even the exclusive, track focused and last hurrah XKR-S GT, were equipped with what I believe is the broadly the same 6 speed box. The presence of the puck gear selector is the most obvious modification but there were certainly changes to control software and presumably gearbox internals to suit the characteristics of the 5 litre engine. Oh, as far as I'm aware the 5 litre was/is a new engine not a 'bored-out' 4.2 (which itself is already a 'bored-out' 4.0 litre), the most significant change being direct injection into the cylinders for improved performance and economy.
I have an 4.2 XKR 2008 Portfolio, am a member of the XKEC and a bit of an XK nut!
Any XK is a great car, just smiles and no regrets
Cheers, David