Glancing at his gear wish list, it looks like he's more into action than astro. An A7R is 2500 less in the budget (camera + EF adapter). Personally I would love one for portrait and landscape work, but I can not justify the expense. I suspect I'd get more use from that tamron 150-600 and a new tripod.
I'm actually pretty into astrophotography. It splits my budgets now. The A7r, along with pretty much any Sony camera, Nikon camera (with the exception of a couple that use different sensors), and a lot of other cameras that use Sony sensors (i.e. Pentax) are all pretty poor choices for astrophotography. Those manufacturers all mess with the image signal pretty heavily.
They clip the black point, rather than using a bias offset (Canon uses a bias offset). That causes two problems for astrophotography: By clipping to the black point, you simply eliminate a lot of the dimmer background stars entirely, they are gone from the signal, unable to be retrieved; They make it difficult to use standard bias frame calibration techniques to remove any noise caused by sensor bias and recover those dim stars (which IS possible with Canon cameras.)
Sony/Nikon/Pentax/etc. also tend to apply noise reduction to the RAW signal in hardware...an unconfigurable noise reduction, that's just always applied. Having total control over noise is a pretty critical facet of astrophotography...the vast majority of images you create for astrophotography have image data only in the lowest echelons of the signal, stars are the only things that have levels throughout the signal. While you can do some pretty amazing things with the D800 at ISO 100 when it comes to lifting shadows, that's nothing compared to the kind of lifting you do in astrophotography. The D800 can be lifted about six stops. In astrophotography, your often lifting by a lot more than that...to really pull out dust lane detail and dark nebula detail and things like that, it's common to lift things by an equivalent of 10-15 stops! Not even the great D800 or any other Exmor DSLR camera can handle that, in part because of the black point clipping, which is throwing away a couple/few stops of potentially recoverable information in the first place.
A proper astro CCD camera has at least 18-19 stops of dynamic range, and usually well over 20 stops. They are thermally regulated (anywhere from -40°C to -80°C Delta-T from ambient), which nearly eliminates dark current noise, generally have relatively low read noise, usually have much higher Q.E., and usually have larger pixels (smaller astro CCD sensors usually have around 5-6µm pixels, larger astro CCD sensors usually have 9-24µm pixels; FF DSLRs tend to have pixels in the 6-7µm range, and APS-C DSLRs are now around 3.5-4.5µm). Since astro CCD sensors are also most often monochrome, and you usually image in LRGB (luminance + RGB), you can produce images with much stronger signals than you can with bayer-filtered DSLRs.
So, while I'd like an A7r for my landscape photography, it is actually one of the worst possible choices for astrophotography. I do landscapes sometimes, wildlife and birds most of the time, and astrophotography every time there is a clear night. Since Canon cameras don't mess with the image signal nearly to the degree that other manufacturers to (they do some response curve tweaks at certain higher ISO settings, but I usually image at ISO 400, which Canon pretty much leaves alone), and since the 5D III can be used for landscapes (it has a very respectable pixel count and frame size for that), wildlife and birds (it meets my minimum expectations for rate at 6fps), AND can be used for astrophotography, it's a far better investment in the interim (especially with prices hitting $2700 pretty regularly now.) It may not have the DR of the A7r, but it is a vastly more versatile device.
If it wasn't for the astrophotography, I'd get a 1D X. By getting a 5D III, that leaves me plenty of cash to invest in a proper astro CCD, a filter wheel and filter system, and a few other accessories.
So...given how versatile Canon's DSLRs already are...do they really need to become a Sony clone with their new sensors?