Need some advice Nikon d750, d800,d810

[Solarflare]

One thing I've always wondered about is the 1.5 stop low light advantage of the D810 over the D7100. Does it hold up when cropped down to APS-C size?

No.

The reason why large sensors are considered to have better low light performance is because one views the same output size.

DxOMark for example reduces the resolution to 8 Megapixel uniformely, independent of the resolution of the camera, before evaluating the noise leves to decide how they rate the High ISO performance.

Thus, if you use the D800 in APS-C mode, its performance is exactly the same as that of a 15 Megapixel APS-C sensor.

Artificially cropping (or digitally cropping) has absolutely no effect on ISO ability or anything relating to the sensor. All the DX crop mode does is to save a "cropped" image rather than the FX image, thus saving space and having smaller files (mb wise) which allows the buffer to handle more.

You argue on a per-pixel basis, but the per-pixel performance of the D800 is not the slightest bit different than of that of a 15 megapixel APS-C Sensor of the same technology.