I don't have the Sightmark, but called someone who does (Sightmark technical support). I nice gentleman by the name of Neal said he would measure one and give me call back. Neal measured the distance from top of rail to bottom of magnifier as 18mm (doing the math, that's 0.7"). Just for my own edification, he measured from top of rail to center-line of the optic at 38mm, which I believe is what I measured on my EOTech 517 which does sit higher than the XPS models that don't have the 7mm integral raised base. In fact, I believe the 517 and 512 are identical except in the location of the control buttons. The 517's are on the side which are a little easier to reach if you are using a magnifier.
According to the Magpul site, the height of the MBUS Gen 2 when folded is just over 0.5", so there should be just enough room to squeeze underneath. It mentions this if you click on "specs."
One last caution though.... If you have the standard length rail on your AR, you may run out of space on the rail for all three. Even with the 517 mounted on all the way forward I don't have enough room on the rail for the magnifier and Troy Industres BUIS. This is due to the mounting ring having to be centered on my Mako magnifier (similar to the Sightmark). The Aimpoint and EOTech ring goes on the front of the optic so this leaves more room behind the mount for the BUIS. If I really need the BUIS on-board, I think I'll need a cantilever mount. For now, it doesn't bother me that much.
I hope this helps if no one else who actually has the Sightmark chimes in.