Forward mounting Eotech
The Eotech won't co-witness with standard height irons if placed on an elevated rail. IIRC, mounting the Eotech in such a way will place the irons in the lower 1/3 of the HUD. When both are zeroed to the same distance, the reticle will not line up with the irons.
If you have flip up front and rear BUIS, lack of perfect co-witness probably isn't a big deal.
Another option would be to spend a little more and add a KAC Free Float (FF) RAS or KAC RAS II.
Either one of these would add a solid forend mounting surface for the Eotech while remaining the same height as the integral receiver rail. Well--The FF RAS would. The RAS II has that hump where it attaches to the upper receiver.
Bushmaster just came out with their own FF tube with integral rails. It's about $100 cheaper than the Knight's, but too new to find any reviews/evaluations on. Looks solid. If they're machining it themselves it's probably a pretty nice component.
I've tried my Eotech 552 (AA model) mounted forward of the upper receiver on a KAC M4 RAS--Made the rifle a bit nose-heavy, IMHO. Balance and ergonomics (access to the controls) is better *for me* with the Eotech mounted as far forward as possible on the upper receiver's rail.
RE: Eotech 2d reticles. Something that I noticed that I haven't seen anyone else mention--because it is a hologram, the Eotech reticle stays the same size regardless of the eye's distance from the reticle. This has the interesting effect of making the reticle appear to grow larger as the distance between the HUD and your eye is increased. As the HUD "shrinks" into the distance, the holgraphically projected reticle remains the same size, leaving less and less room between the 65moa ring and the outer edges of the HUD window. Probably not of much concern considering the range of distances available on an upper (even with a forend rail)--But something to keep in mind.