Input lag was tested using DisplayPort and DVI, and both were the same. The dE measurements were done using the DisplayPort to DVI adapter (this is discussed more on the next monitor review being published) as I do those in OS X, but I do the lag measurements in Windows using SMTT. SMTT also does a full pixel-on to pixel-off to measure response time, since that should be the worst case scenario, whereas most panel vendors (and therefore display vendors) use a GTG response time that might not be nearly as stringent.
I really hate that the lag times might be off, but I really don't have a good way to test monitors of this resolution for lag. CRTs don't do 2560x1440 natively, and the CRT and LCD have to run at the same resolution to be tested properly. There is a nice new lag device that someone made for testing TVs, but it caps out at 1080p since it's designed for TVs and HDMI as well. Until OLED is available, and one with zero lag and instant pixel response, this might be the best I can offer unfortunately.
You can also look at the charts and see that only the 27" HP, with no scaler, has less lag for 2560x1440 displays. So if you can test out another 27" display and it works fine, the Nixeus should work fine I would assume.