I think the point is that wider rims mean wider tyres. On the 55 the rear tyre is 255/35/R18. I think that's about as wide as I would want to go with the standard 9.5J rear wheel. With a 10J wheel, I'd be happy to put a 275/35/R18 on there and leave the fronts alone.
In my opinion (and I know others differ with me on this) the car lacks rear end traction and grip. A roughly 8% increase in contact area (I think I've calculated that correctly) will help, even if it's just a little bit. Plus the rear tyres look great now, a bit wider will look even better