I guess it all depends on what Aimpoint Rail Grabber Mount we are speaking of.
I know that some people think the aimpomit mount that comes "free" with a COMP-M, and such, is a little too low for co-withnessing, but shooting it mounted thusly seems ok to me.
However, I long ago hacksawed the front sight tower off the upper I base this opinion/evaluation on. So, I just tried this on a barrel with a normal sight tower, and man, you can really see a lot of front sight tower in the Aimpoints field of view.
By switching over to our Low Profile Aimpoint Mount (P/N: 99430) that is about a 1/4" higher, this appears about perfect for not having the front sight tower image bother you too much out in front, and for the co-witness function. By the way, the centerline of this low profile mount is about 1.10" above the top surface of the rail its mounted to.
Our other Aimpoint mounts are surely too high to mount on the "hump", as they are 1.57" high, which is where you need to be if you want your Aimpoint on the upper receiver's original rail surface, or the front (same height) rail of the RAS II.
An advantage of these "too high" mounts (P/N: 98512 & 98073) however, is if you are possibly going to shoot with a gas mask or respirator on, then moving your Aimpoint to the "hump" fro m the lower rail sections may be a good idea, as its higher line of sight may match you now higher cheek weld.
I am a big believer in "clearing the deck" when it comes to potential sighting & field of view obstacles. That's why my personal guns have had the hacksaw treatment. Since I have a RIS or an M4 RAS of one sort or another on these guns, I use our MWS Folding Front Sight out on the end of the top rails. I lose a litle iron sight radius thusly, but this is my back-up mode. My Aimpoint on one, and my Elcan on the other are the primaries. ANd with 56 year old eyes with a major stigmatism in my master eye, I need all the help I can get.
ColdBlue sends...