My rear rim is a 5.5. The 190 tyre just clears the swing arm. It looks awesome. You can hardly see daylight there, but after a fair few miles there were no rubber marks on the swing arm when I took the wheel off.
I take the point about bikes generally going better with 180s, but in practice I've found this only applies to sports bikes. The GS is relatively immune to the slow steering effect of a 190 because the bars are so wide. I suspect a much bigger factor with the GS is the different profiles used by different tyre manufacturers. E.g. with the Bridgestone Trailwing the bike falls into corners like lightning. But with Avons (my current pseudo-trail tyres) it's slow, steady and progressive.
The only road tyres I've used on a GS are the BT-57, Z1, Azaro, 207 RR and 207 GP. The only time I've used a 190 was with the 207 GP, and the steering couldn't be faulted. I'm a firm believer in 207 RRs and am always relieved to come back to them, especially when the roads are wet. Next time I replace my 207RRs I'll go for a 190 again, just for fun.