Not sure if this applies to you. Tyres in the following sizes should all be the same rolling diameter within reason. 205/65/15, 225/55/16, 235/45/17.
If your speedo is out with your stock 215/60/16, I wouldn't be surprised if it is corrected when you go to the 235/45/17's.
Give it a try before you spend the $$$ though.
Aus LS1 Moderator.