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

Close up photos will reveal workmanship of the mods.
If lack of reference point (you dont have friends who have a stock/similar unit) and resale value is an issue where you are then stick with a verifiably stock unit.
The LS15 is not that expensive. By the time you send it to Audio Research for check and retube and stuff you might as well pay a premium and buy a stock unit.

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.