Superpsu,
Here's my take on the wider rear tire topic. I happen to love the look of a wider rear tire on most any bike.
I replaced the rear wheel on my S4RS with a 6.0 from a 5.5 so I could make the change from a 180 to a 190 appropriately. The point being; if you don't increase the wheel size you're really not going to notice a difference in width from a visual stand point. The lip of on both tires contact the rim at the same width, the extra rubber on the 170 will just be pushed upwards, hence creating the steeper profile. So the 170 really won't look wider as it will look rounder or have a higher arc.
Finally, without increasing the wheel size you will probably change the handling of the bike in a negative way.