Lynn- First let me express my :bow :bow for how you keep your XJC so nice without a garage! Just mind-boggling.
I'll let someone more knowledgeable comment on the newer materials. I used Evolution IV (as I recall, maybe III, long ago) on black, white and silver cars. Every car I used one on got scratches from it, no matter what I tried. Otherwise they were OK, except when very wet (sorta heavy, sorta messy, water that seeped through mixed with dust, etc). They did require a larger washing machine than my home one.
Some negatives to consider:
What to do with the cover when its wet and dirty?
How to avoid scratching the car without washing/QDing all the time?
What about dressing transfer from the wheels (and vinyl top if you replace it)?
Those negatives against the self-evident points in favor (you sure don't want to cook that interior). :nixweiss
FWIW, I might start covering the Volvo, but I don't care too much about a few more scratches on IT.