I'm running an 11-speed 14-32 cassette, built from a 14-28 and 11-32. It's perfect for me -- I get close ratios all the way up to 25 mph, and still have low gears for steep climbs. I have 34/50 chainrings.

Actually, the 14-28 would be fine by itself if I didn't have some very steep climbs.

Details in this BF post.

A few downsides:
I spin out at 29-30 mph, which is acceptable.
I have to shift 4 or 5 cogs when shifting the front ring. But that's fast and easy with Di2.
The small ring runs out of gears around 16-17 mph, instead of around 20-21 mph with an 11-32.
It's kind of expensive -- I've only found Ultegra 14-28 cassettes.

Lots of close shifts: