The Nexus 5 has GRAM!
I stumbled across a review which specifically mentions and tests Panel self refresh (PSR). From their browser battery tests it looks like they are getting just under 7 hours without PSR enabled and just under 8 hours with PSR enabled. It provides around a 15% -25% increase in battery life depending on usage. I have no idea how they figured it out, but it looks like it's there.
I can't post outside links unfortunately but the review is at tweakers.net