You gain 5% rolling radius which is quite a bit, can make the turning circle bigger and you'll get more road noise/more chance of aquaplaning so not all pros. If you want the pick of the new UHP tyres they all start from 18" anyway.
I found that out by ordering a full set of Continental tyres for my ST-2 online at work without paying much attention thinking I was getting SportsContact 6 (only available on 18" and up on checking properly) when I really ordered PremiumContact 6
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. I had a bit of a panic at first because I had PremiumContact 5s on my old Focus and they were okay but really soft, but reading a few reviews suggested the PC6 was designed to replace both the SportContact 5 and PremiumContact 5 so I stuck with them.
I haven't worn them in fully but the ride is better and quieter than the horrible Bridgetone RE050As I had before, they're XL rated so don't have wobbly sidewalls like older Continental tyres. The dry traction is good and the steering feels nice and pointy, no idea what they're like in the wet but the RE050As had awful wet grip so I can't see these being any worse. So far so good.