Does moving away from the factory profile and width of 215/45 affect your fuel economy or suspension? If so, how greatly are they affected? I am thinking of getting some new 17'' tyres and I am thinking of going with 205 or 215/40 as I prefer to see smaller tyres. Do I compromise anything by doing this?
Why would you want to go with a narrower or lower profile tyre? What do you intend to gain from this? Most people want to fit wider tyres which I can understand. As Noel said all it'll do is put your speedo out. 40 series will also make the gap between the guard & tyre bigger making the car look stupid/higher unless you have coilovers to wind down. If you are still intent on changing them make sure you do all 4 or you'll end up with ABS troubles.
205/40 will change final ratio (virtually). Car will launch a bit faster but your highway RPM´s will go up a bit. So you´ll lose a bit MPG-wise. Also your car will look silly. If you want narrower tires, go with 205/50. it´s about same diameter as 215/45.