I like the diesels so won't bash them. But even an auto 4.0 litre should still be quicker than a chipped 2.5 as there is still 20-30hp difference and less weight. Add in better gearing and a more efficient drivetrain of a manual and it wouldn't really be a contest. Not if the person can actually drive it.
The p38 weighs a tad more than a classic. And I've personally seen an indicated 125mph from an auto 3.9 and they are quicker than a 150hp tuned Tdi 90. So a 150-160hp diesel p38 is not going to be playing ball.
Well i did say not far behind. Weight depends how fat the driver of the petrol one is. Diesels are quite capable of doing 30 MPH plus sat at the speed limit. Which is 70 MPH max in the UK i believe. IOM excepted.