Only you can decide whether you're far off or not but here's my logic in detail
Units for this are relatively un importnat for the maths of this so for the purposes of this I'm going to measure in carrots as it avoids any transatlantic metric imperial clashes
a 180/55 tire has a 180 carrot width and a 55% aspect ration to achive its height whihc is 55% of the width
so its height using the aspect ration is 0.55 * 180 = 99 carrots
For the 170/60 tire its height using the aspect ratio is 0.6 *170 = 102 carrots
so the 170 tire is taller by 3 carrots
now the problem is that as far as I know there is no standard wheel definition in automotive (there is in aero) and things like rim channel width and depth can and do have an effect on tire profile but benchmarking two tyres of the same model and brand the 170 will always be taller.
now for my slightly more subjective logic: If I fit a wider tyre then I will increase my contact patch but if my height remains the same I will reduce its radius - i will put more tire on the ground because my lateral measurement of my contact patch will get fractionally bigger...... but i now have a more gradual radius and more tire on the ground so it will require more force to turn. I've also added more rubber and there fore more mass to the rear wheel increasing its propensity to stay in one direction when rotating - I've also added more unsprung weight so given my suspension a harder time.
What you are talking about is fitting a wider and lower tire so you would increase your contact patch and decrease your radius and although the differences are small.
Now take those small differences and start to look at weight distirbution. again a small change but you've lowered your rear ride height and therefore fractionally shifted your weight distribution back on the bike.
Now you can adjuct this potentially by adjusting your rear ride height but keep in mind we have no ride height adjustment on these bikes only pre load.
(Pre load should be used to set sag and that is all, any adjustment after you have that right for the bikes load is just a measure of your capability to screw up your bikes handling - as described to me by an ex formula 1 suspension engineer)
You've also made a smaller change to rake and trail - essentially you have kicked out the forks slightly - not enough to fit ape hangers yet but slightly none the less.
So to my mind fitting a wider tyre means you will require more force to turn the bike, you will have less weight on the front of the bike and you will have made you're steering more "sloppy"
all of which are fractional but combined are quite effective.
WHich leads me back to my view on tyres for this bike - go for the highest profile recommended fit tyres you can get and then ride the hell out of it - I like my bikes to turn quickly and controllably - quickly is a relative turn on a sports tourer as you don't want it to bite you in the butt. From a personal perspective I think Bridgestone are the work of the devil and object strongly to the fact they don't make a 170 rear. But then I've never liked them on any bike.
I still can't get over how horrible Michelin Pilot Roads were when I had them (Pilot Road 1s) and have always found Pirelli/Metzeler, Conti Road attacks and Avon Azaro/Storm tyres work well for me although not always on the same bike.
So thats my reasoning - more than happy to be told I'm wrong but I struggle to find a positive argument for the 180 rear
