Unlike the Metabones Speedbooster which acts as a focal reducer, their M -> E adapter is purely mechanical (there isn't enough space for a focal reducer) so the focal length and field of view are unchanged. This is a good thing as the A7R has a full-frame sensor so you get the correct field of view and the image circle covers the sensor.
Newer Leicas are 'designed for digital' however the digital Leica bodies have thinner filter stacks over the sensor than the A7R and specially designed microlenses to handle the steep ray angles you get in the corners with rangefinder lenses.
Performance with Leica M mount lenses is thus a bit hit and miss on the A7R, it's worth checking reviews of specific lenses before purchasing. The is also a mod offered by a company called Kolari vision which installs a thinner filter stack for better performance with Leica and other rangefinder lenses:
http://www.kolarivision.com/thinfilterconversion.html
There's plenty been written on the subject of M lenses on the A7R, if you google you should be able to find tests of lenses you're interested in. Here are some good links for starters: