I used 180/55's on my 800ss - no probs there at all. I've not notice any issues with steering. The 5.5 rim is a little wide for 170's in my opinion looking at how they wear.
Mood: 900 went 999 gone, Multi came and went, 749 Nero now
I posted a while back how impressed I was with the pilot powers, dont think you'll need the 2ct things. Put it this way. the powers were the only tyre going on to my 999. This months superbike has a test of tyres and the powers came out on top You will not go wrong with them