I'm not sure why you would want to put the stock 17"s back on when you have the lighter and much better looking and handling 18s.
There are no real good max performance summer options in 215/65R17, 225/60R17 (stock on manual cars), or 235/55R17 with their massive flexible sidewalls.
Stick with the 18s. They are lighter than the 17s, better looking and there are plenty of good tire options in the 235/50R18 size. I am not sure what problem you are having with the OEM Pirelli PZero Neros but there is no way your car is slower with them compared to the stock 215/65R17s.