ARC LS15: did they ever use Mundorf MCaps on it ?

To my knowledge Audio Research never used Mundorf caps in production. They used Rel,Infini and MIT in older units. Hope that helps.

I would always choose a Mundorf Cap'd unit compared to a stock one.

yes, but for me not before listening to the stock one (that's a personal opinion, obviously); and then again, I would neet to be sure about who and how worked on the preamp.
moreover, as I already said, if a seller lies on one self evident fact, he may lie on anything...

anyway, in the end I decided to let the LS15 go... now I saw a pair of LS16 (one mkI and one mkII); see related thread :-D

but most people tell me that neither an LS15 nor an LS16 (regardless of mk) would be a significant upgrade for me.
most people tell me to go for an LS22 or LS25 or, at least, LS17.
they may be sure right, but that takes 1000 euros more...
I think I have to think it over for a while.

"the key words are "as long as it's done right"."

It was done right when it left the Audio Reseasrch factory. Have you ever seen the facility at Audio Research? If you did you would never trust some guy in his garage or little shop with no real knowledge and no equipment to speak of other than a soldering gun. As far as I am concerned these guys are shysters just looking for a way to make a buck by leading you to believe they are going to enhance the sound of your already excellent sounding equipment.