As for ammo and accuracy, one of the neatest things about this rifle (besides how naturally it points), is that it performs almost equally with anything you feed it, and it has cycled every ammo brand, both milsurp and civilian, that I've fed it, and groups about equally well. I haven't benched it, but offhand I'd say that 1.5" - 2" groups wouldn't be out of the question. Good enough for government work, if you know what I mean...
The DSArms mount has a good reputation. I've got one of the early DSA mounts on my carbine now, with a TC short-tube illuminated reticle 4x scope in low Weaver rings (the ocular just clears a folding L1A1 rear sight). (Great scope - I've owned several. Of course, they don't make them anymore... ;>) It's a good combination: lines right up, with an excellent cheek weld on the standard 'humpback' stock. It made sense on the heavy barrel (which I just sold to pay off some bills;>), but seems a bit too much for a carbine. A lot of folks seem to mount red dot or holographic sights on the carbines, using the same or similar mounts.
And with quality ammo, what kind of 100 yard groups is a good FAL capable of?
I've heard anything from 1.5 to 3 minutes of angle for a real rifleman with a quality FAL and good ammo. Personnally, I would expect 'service rifle' rather than 'target rifle' accuracy - I would bet, for example, that most FALs can not compete with most M1As. Maybe someone else here has personal experience...