This test shows some to the aspects of using the same Sony SAL70-300G a-mount lens and adapting this lens to the Sony a7RII using the Sony LA-EA3 and LA-EA4 adapters. Tests were @70mm, f6.3 and color temp set to 2650 degrees in ACR.
The LA-EA3 is essentially a spacer with electronic pass-through and it metered at 1/8 second shutter speed.
The LA-EA4 has a semi-translucent mirror in the way and it metered at 1/6 second with a similar histogram, which is the 1/3 EV differential I expected.

This image is a 100% cropped comparison from these two adapters. The only thing that is not self evident is that I did a gentle tweak to the LA-EA4 image on the right in Photoshop, exposure-wise, to normalize the whites.
The total exposure variance is still significantly less than 0.5 EV…I think it was 0.05EV plus the shutter speed variance of 0.33EV.