All tyre websites recommend 205/40/17 if using 17 rims.

What will be the effect of running 215/45/17...

Im guessing it will feel mushy when changing lanes at speed due to the higher side-walls and perhaps a speedo that shows the car travelling slower than you actually are.

Am I corect and what other differences will I notice?

Thanks

here you go mate have a play
http://www.tyresite.com/tyrecalc.asp