http://www.rimsntires.com/rt_specs.jsp This site is a great tool for these types of questions. Type 215/45/17, then play around with random sizes, and you can see how much shorter/longer the sidewall will be. It also shows you fitment issues, as well as % and speed your speedometer will be off. I run 235/40/17, and to me it is a good compromise between performance/value, any wider and the price goes way up. I see the best prices with this size, as well as 215-225/40-45. You can do 235/35/17, it will be an inch shorter overall(half inch top and half bottom), and you speedometer will only be 5% off. 245/35/17 will give you good performance, 22mm shorter sidewall than what yo have, and only 3.5% difference with speedometer, yet this size is likely 15-20% more. This site should let you see everything side by side and give you a good idea. I would be worried about going to small, not sure how it would affect performance, tire ware, suspension geometry, and not to mention that you are lowering the car too.
One thing to remember is that all tires/manufactures can have slightly widths, diameters, contact patches. I believe that most summer high performance/track type tires will appear to be smaller profile than all season tires, which have so much going on tread wise. What kind of tire do you want, high performance, value, chinese cheapo, comfort, long tread ware?