I am very curious to know how using smaller rims even with summer tires would affect the dynamics and range of the car. Since the 21" rims are less efficient than the 19", would 18" be even more efficient?
Assuming they clear the brake hardware, 18s if not heavier, should be more efficient than 19s, due to having narrower tread width and less rolling resistance.
235/50R18XL should be the closest size to replace 245/45R19XL.
I'm not 100% convinced they will fit properly, however.