I bought and can confirm this one worked fine from the OP -
https://www.amazon.co.uk/gp/product/B013G4EXMM/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1
available in the UK, whilst there were no issues the picture in the rift didn't appear to be any different, not that I could notice anyway.
What did make a difference was simply plugging my monitor into the HDMI port I usually run the rift on, which enabled access to the Nvidia Control Panel for that particular output on the card, go through the settings to add a touch more contrast and vibrance and of course set the output to full range, then just pop your screen back on it's normal output and plug the rift back in the "tuned" hdmi port.
I might be wrong but I'm thinking those that are having a noticeable change when using the DP to HDMI adapter just so happen to have full colour enabled on that particular DP output, I'd suggest at least trying that method before buying an adapter, chances are you don't need one.