here is a few pictures for you
Even though I say for the 5DSR you have to be at 1/2.5x to 1/4X FL to get 'tack' sharp image, I was able to get at 15 feet what I would accept as a tack sharp image 1/160 for the 135L. See below what the setup looks like and then the massive zoom. Look at the music notes on the card to make your judgement.
Because of exposure, I just was not able to be fair to the A7R2 as a comparison so I stopped down the lens to F2.8 to reduce the exposure. I still had to pulled exposure down by 1 stop and reduce Highlights by 92 in LR. As you can see, at 1/15th with the A7R2, I was able to get basically a tack sharp image with the 135L - to me that is crazy.
Because the exposure is really out for the A7R2, it isn't appropriate to compare colours, etc. You can see, I think, the higher megapixels of the 5DSR - but the point of this post is that, even while I wouldn't use 1/160 SS on the 5DSR with the 135L if it was handheld, I was able to get it sharp - that said, I can do the same at 1/15th with the help of IBIS and less shutter shake of the A7R2. I also noticed, after posting the pictures to flikr, on the 5DSR you can see CA around the edges of the fence hole and edge of the card @ F2 (no surprise given the situation and being an older lense). I went back and in LR this is easily removed by clicking CA and then reducing magenta -2. I tried to take the pictures relatively quickly back to back but small light changes do happen - but either way, it is interesting that the CA does not appear in the Sony (might just be comparing at F2.8 and the Highlight reduction that was necessary given the exposure but not completely sure)?
The 5DSR crop (1/160, F2, ISO 100) - Highlights reduced -20 in LRIMAGE LINK: https://flic.kr/p/yMj5MS 5dsr_vs_a7r2 by Paul O'Neil, on Flickr
The A7R2 crop (1/15, F2.8, ISO 50) - Highlights -92, whites -29IMAGE LINK: https://flic.kr/p/z4UDUB 5dsr_vs_a7r2-3 by Paul O'Neil, on Flickr