Well, you have 2 things:
1. For image quality say at standard ISOs up to 3200 then A7R smoking that 5Dc, also that 5Dc can't go higher than 3200, i can't remember if it can go up to 3200 either, so then A7R wins here no doubt.
2. AF speed, using the metabones adapters sure will slow the focus, but, using that 15mm and 17-40 you are shooting so wide scene and then you don't need to use AF speed all the time, i don't think you are shooting fast moving subjects, so that 5Dc will not be that much big help, i know that 5Dc maybe faster AF than A7R, but think what you shoot with 5Dc using 15mm or 17-40.
So, from above, if i have the choice i will go with A7R, but the way, i have 16-35L and 15mm fisheye and also 5Dc and 1Ds3 and 1DX, so i am experienced with full frame camera and i chose A7R for what i will shoot with 16-35L and 15mm.
Ait, thanks, still in doubt if I'm getting the A7r or the new A7s, since the high ISO performance on the A7r seems good but still so so. On the other hand, the 36mp is pretty tempting :P