Like I said, personally at 70mph I would feel better in a car that isnt Cat D, and thats a personal choice and happy to pay the extra couple of grand for, especially if I am not the only person in the car. Like you say 'if its done properly' so, if, it's not done properly then there could be a problem. Its personal choice, which is probably part of the answer to the original post, personally for me I saved up and bought one that had a clean bill of health with no previous, I could have short cutted to a Cat D but because of aforementioned reasons about peace of mind and also with half an eye on selling in a few years and being able to move it along more smoothly than a Cat D.
The pertinent point you made above which in effect contradicts itself in that 'the myth of shocking CAT D's are a bit of a thing of the past, everyone is still hung up on what their parents grew up with and there folks warning against dodgy cut and shut jobs etc' it was not so much a myth but quite true a few years ago but as a myth it cannot be a thing of the past if as you say people are still hung up on it. I am not saying Cat D's are not good cars, what I think is that generally people are still avoiding them, so come sale time you are at an advantage in moving your car along quickly if it is not Cat D.