I had one in a 525D, I actually thought it was 'okay'. I had it chipped and then it was 'alright'
I'd certainly have thought in a RR it would be comparable performace wise with a Discovery TDi. Although admittedly, I've never owned, nor driven a P38 with any engine.
my mates P38 ****es me on the road... well i say ****es... definitely top end, its noticably better on road, he takes corners quicker and more comfortably, it is however noticably worse off road!
From what I've seen of P38s (with the right tyres, and suspension raised) they were extremely impressive off road. I thought the challenge was just to keep them working