Very interesting question - I have a shitload of stuff that I'd like to do (in my mind would be upgrades from factory) but I'm (like the OP) worried that should the worst happen and an assessor looked at upgraded springs (H&R) for example, take's a jaundiced look at the car.
My car is going on 14 years old and is factory but needs stuff taken care of at this stage - where do they draw the line?
How does it work in the states? Seemingly you can do anything to a car over there - it's a way of life almost..