I just went through this headache. Here is the play by play and the outcome...
Bought vortex 3x, doesn't fit on upper (not rail) with BUIS and Aimpoint in QRP2. There is just not enough real estate on there as the vortex magnifier is mounted in the middle of the tube. Side note, Vortex is flip left, this sucks for right handed shooters as it blocks your peripheral vision when not using the magnification. You can reverse it to flip right but then the release is forward on the mount and not quick push button from the rear. The glass was nice but they need to offer a R or L flip option and mount the magnifier on the front so it hovers over the BUIS IMO. This would be a winner if they made those changes.
Next I bought a used Aimpoint 3x, good deal here on this board. I bought the LaRue LT755S to try and keep absolute co-witness. LaRue doesn't make a cantilever absolute mount for the Aimpoint so I went the with Bobro absolute cantilever mount. Turns out the Bobro mount in absolute actually isn't cantilever as advertised. They also only make a cantilever lower 1/3. Because of this the Bobro has the same issue, not enough real estate to fit everything on just the upper without hitting rail. Bobro and LaRue get returned.
Next I bought the LaRue LT755T and the LaRue LT129 and all is well in the world. This is lower 1/3 co-witness and I am fine with it. Looking back I am not sure why I was so bent on getting absolute co, really no idea...
In all I wasted about $50 in return shipping to the different stores and about 30 days of my life trying different shit. In the end I ate my own words, buy once cry once. FYI - you could buy direct from LaRue the packaged Aimpoint 3x and LT755T for $660 -
http://www.laruetactical.com/aimpoint-3x-magnifier-larue-tactical-qd-pivot-mount. I saved about $200 by buying the glass used and adding the mount. [STRIKE=update]Don't forget, if having BUIS is important with you the stock QRP2 mount will have to go to give you the room for everything. This will work and lineup with the previous magnifier -
http://www.laruetactical.com/larue-tactical-cantilever-compm2-lt-129. [/STRIKE] - Update, I didn't actually test just the Aimpoint magnifier in the LT755T with the QRP2 mount. I will take a look tonight and get back to you but I feel that it won't fit as the QRP2 mounts near the middle of the 30mm tube on the PRO thus making it hang back, hitting the magnifier.
If you didn't already buy the pro you could save some money too. They offer the Pro in the LT129 fro $416. I got my PRO from AWS here on this board out of NH and it was a great deal, better than anything I've seen on the web since. But, if I had to go back I would buy it from Larue in their mount. It would have saved me over $100 buy getting it in their mount from the get go.
Pic from Larue but setup looks like this now -
Good luck!