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

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.

Rrog, Do you work for Audio Research or sell their gear? Please kindly reconsider your comment.
You may be stereotyping a whole lot of well respected boutique component manufacturers who work with honest and knowledgeable modders out there.

Johnsonwu.....Amen with last post.

To the OP, If it has a GNSC badge on it no worries. Steve Huntley's (and his team) workmanship was without fault. He used premium parts as well IE; Vcaps, Vishay, Caddock, and others. I had many units modded by him including (but not limited to): REF3 LE, REF2 Phono, Classic 120's, DAC7 with astonishing sonic results.
Actually, when you look inside a REF5 SE and REF2 Phono SE you can clearly see some of Steve's signature work that i have in my units particularly the power supply upgrade.

.......and to the modder detractors, the launch of ARC was orchestrated by one of the most renowned modder of its time pre 1970.... William Z. Johnson.

Yes, a modded unit's resale value would typically be less than stock although i have sold a few ARC GNSC modded units at premium prizes over the years. There is a niche market for modded units including ARC's.