First, here's a little background that prompted the question:

I'm building what I will call a CAR-10T. Don't ask me why because I'll never give you a straight answer. In short, the rifle would be intended for close quarters use with maybe a few outside shots.

Basically, the rifle is what it sounds like:

Top End: An AR-10(T) carbine top end with an A.R.M.S. S.I.R. System (with the fixings), a JP Recoil Eliminator, a Laser Devices MR-2, Elcan, etc.

Lower Receiver: Eagle Arms lower with the ArmaLite two-stage trigger, a Rock River Arms Entry Tactical Stock with a KMC Stock Extension Sling Mount, a custom buffer (154 grams, 2.5”), and an SR-25 Op Spring (modified to 29 coils).

Optics and sights remains an unresolved issue. Currently I have an Elcan on the upper with plans to use an A.R.M.S. #40 flip-up rear sight. I can’t for the life of me decide what front sight to use: A Bushmaster flip-up (with .040 blade), a fixed A2 sight (with .040 blade), or something else entirely.

The question is: How important is the co-witnessing of optics? Do I want to trade down to an EOTech (1 MOA dot) that will allow me to co-witness my backup sights and my primary sight? Or do I keep the Elcan and use the back-up sights as back-up sights?

Such a dilemma, please help me!

That is going to be a heavy CQB weapon. Why the JP brake??

The EOTech will not co-witness with a SIR system unless you get the #50 bi-level.  By getting the #50, you can forward mount the EOTech on the low rail on the handguard section, while your laser can go on the side rail.  The standard SIR with the single pane rail will be too high for the EOTech.

Flip up front sight is nice, but a CQB weapon really have no need for the flip up front sights.  I don't think the front tower will not bother you.  if it does, just get the GG&G sights later.

One quick advise, don't get the 1 MOA EOTech, for CQB, it will be way too small.  I highly suggest getting the standard EOTEch, which you will still have the fine 1 MOA for distance shot and the 65 MOA circle is awesome when it come to picking up target fast in the CQB enviroment.  

When I chose my mount and scope to cowitness I had a different thought process.  I wanted to shoulder the weapon the exact same way every time, no matter what I was shooting at and what I was using - sights or optic.  To me that is the most important requirement.

Sarge

For the many match and range shooters who keep the rear sight down when shooting and possible nearly never use the iron sights, then IMO lack of a co-witness system is not that bad, as the iron sights are there for backup (or looks).

Still need to have the optical sights low enough so you can use them with a good stock weld, which a co-witness setup tends to generate (plus as discussed a co-witness system requires only one way to hold the gun.)

Where the co-witness appears to come into play, is real world applications, when depending on lighting and/or failure of the optical system (i.e. one pushed the wrong buttons [;)] you might have to use either sight (plus the little red dot on top of the front post just works well).