Just another data point here for anyone who stumbles across this thread:

I ordered a StarTech DP2VGAHD20 from their Amazon store on July 29th, got it today and popped it open, and can confirm that it has the ITE IT6564FN.

In terms of performance, it seems to be able to do up to 2048x1536@82Hz. I couldn't get it higher than that, even though my monitor can do 2048x1536@85Hz directly from the DVI-I port on my GTX 970. This might be because I'm just using standard CRU resolutions, and I need to fiddle with the advanced resolution settings to get 85Hz. I can, however, also get 1920x1440@90Hz to work fine, which means it should be capable of supporting the max resolutions for most other monitors.

I'll also note that connecting the monitor to the adapter before connecting the adapter to the GPU's DP causes Windows 10 to identify the monitor correctly as a GDM-F520, and provides a bunch of high resolutions by default. Connecting the adapter to the GPU and then connecting the monitor causes Windows 10 to recognise the display as the adapter itself (or something) instead.