Well something to take In to consideration then if it's for looks is.
A 215/45/17 tire is about 10mm bigger in dia to a 205/45/17, and a 215/40/17 will be around 10mm smaller than the 205/45/17.
This will mess with your speed reading but that's not to much of a problem as long as you know what way it has effected it so you can compensate
also if you put a 215/40/17 tire on because it is a smaller diameter than the 205/45/17 it will make your wheel gap bigger between your arch trim and tire and visa versa withe the 215/45/17
Plug the sizes in to this link and you will see what the differences are between tire profiles etc
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