An update: I’ve finally gone to 215/45 R17 and I cannot believe how much more grip there is for such a small increase in tyre width, genuinely astounded. I’m on much cheaper Falken FK510 tyres and the grip levels run rings around even the most premium stuff at 205 width, to be honest there could be a bit too much grip for having a giggle! My car is not running a factory alignment though, Rodders did mine (before my width increase). The handling balance is different, it’s a lot less tail happy and far more progressive entering a slide, when pushing hard it just seems to push wide across all 4 tyres rather than understeering, not sure if that’s down to the tyre model or the width increase.
If you are considering going to 215/45 R17 tyres (from 205/45), here’s some figures you may find handy:
5% wider and 1.5% taller diameter (don’t forget the sidewall will sit at an angle slightly so a little extra kerb protection), Rolling circumference goes from 1936mm to 1964mm so you’re regearing by 1.45%, that means the speedo is 0.9 mph more accurate at 60 mph. You also raise the ride height and reduce wheel arch clearance by 5mm.