Upgrade or sell ARC LS-15 ?

You should be able to get a LS-25 for $2000-$2500. I would go this route, personally. I've gone the mod route before, and although it does make an improvement, you won't recover as much of your money should the upgrade bug bite again. Generally speaking you're lucky if you get back 25 cents on the dollar for the cost of the mods. That's not a sound investment. If I really wanted a modded GNSC LS-15, I'd wait for one to pop up on AudiogoN and buy it for about $1750 or so.
As always, YMMV.

Cheers,
John

Here you go Matt:

Audio Research LS-5 mk III

I have no affiliation.