The beloved V8 isn't really a Rover design either, it's an old buick cast off, after it was found America wasn't ready for lightweight aluminium blocks.
Regarding originality, I for one prefer cars that have been used. Original condition, for me, would be brand new as it left the factory, not a restoration to original condition. I mean no disrespect to anyone who restores their car to original concourse condition, but if you've taken everything apart and rebuilt it, then the car's not been assembled by rover. It's been built by the restorer from original rover parts. For me, the history of a car doesn't stop at when the car leaves the factory, it evolves through the life of the car. Who's to say when a period mod stops being a period mod, and when it becomes a modern abomination. I try to use my P6 as a daily driver, and so have fitted uprated lights and electronic ignition. I try to stick with mods in keeping with the character of the car, so no big spoilers and chrome spinners for me
I wouldn't be adverse to a set of Rostyles though. Would that count as a new mod, or a period mod? Maybe in 30 years time someone will view my electronic ignition as a period mod?
Personally, I think there's a fine line between modification and spoiling a car. I'd welcome anything that allows me to use my P6 every day more effectively. So for me, diesel conversions are in, as is EFI, providing it doesn't spoil the character of the car though. Personally, I'm not after anything that would make it especially faster. I've got the XJR if I want to scare passengers
As for handling, if I want something that handles like an elise, I'll buy an elise, but something that tightens up what's there isn't a bad thing.
I think it's a mistake to assume the factory spec P6 is what Rover's engineers intended. I work as a designer in an engineering/design office, and the designs we produce are invariably a compromise based on what we can achieve in the time, and what budgets will allow. Given free reign, the designers and engineers would produce a technically superior product, you'd just have to pay more for it. I like the idea of mods that bring the product more in line with what the designers and engineers were aiming for.
Out of interest, on my daydream list of engine swaps would be:
BMW 2.5tds straight 6 (if it fits length wise). Cheap, readily availably, improved fuel economy, and refined for a diesel.
Jag V8. It's a cracking engine, far more powerful than the old Rover V8, and cheaper for the same power. Don't know if it would fit though. In supercharged form it would be a rocket ship (if the shell held up) but even the basic 3.2 would shift.
TDV6. I have no idea how the home mechanic would go about fitting this, but it's a great engine in the XF, I've driven one and it really didn't feel like a diesel. I think it would really suit the P6 character as a compact executive saloon.
As for modding the stock engines, I like the idea of EFI on a 4cyl. Rover experimented with this themselves, and I think it would greatly improve the usability without really effecting the character of the car, unless you count heavy fuel consumption as a character trait worth saving.
Tom