I am doing something similar... I am going more for of an all-around battle rifle than long range precision, so I want to keep it light.
I am going with a BCM 20" Gov't profile upper, which is very lightweight compared to all those heavy barrels everyone is recommending, along with being chrome lined for reliability and corrosion resistance. I also want a collapsible stock... I am going with the Vltor E-mod. It is not as light as a CTR or MOE, but it is not as heavy as a PRS or UBR, and it gives you a lot of storage. Plus it is very solid. I figure it will balance well with the light profile barrel. I plan on using a 12" MI-SS free float tube on it, which will help keep the weight down and allow me to use rails only when and where I want them. I will leave the fixed front sight on the barrel, and put my sling swivel on the FF tube... that way I won't be pulling my front sight when I'm slung up and using irons. I will top it off with a TA-31 RCO A4 ACOG, and I will use my KAC 2-600m flip up rear sight... that is, if I get it back in one piece from my friend who is borrowing it and taking it to Afghanistan!
Yeah, you don't see a lot of rifle-length AR's set up with collapsible stocks. I guess most guys either want a close quarters carbine or a heavy long range precision rig. Me, I want an all-around battle rifle that is light and maneuverable enough, but that has the longer max effective range and better sight radius of the rifle length setup.
Now for the question about using a collapsible stock for long range marksmanship. When it comes to precision shooting, consistency is key. You want to always make sure you have the same sight alignment and sight picture for every shot. The reason a collapsible stock can present problems is because if you change the length of the stock, it moves your head and changes your sight alignment/picture. You can do just fine with a collapsible stock though... the key is to find which position works best for you and then always use that position when you are shooting with a particular optics setup. Stay consistent.
Lots of AR shooters put their nose on the charging handle to stay consistent. With a collapsible stock, I like to find the stock position that puts my nose on the charging handle when my neck is stretched out as far forward as it will go, and then mount the optic so eye relief is perfect when I'm in the prone position with nose to charging handle. You can then mark that stock position, or just remember it.