Re: 225/45-18 & 255/40-19
The issue as I see it is being pretty limited to about a 25.5" tire in front and about an inch larger diameter in the rear (issue in rear is that the wheel/tire moves forward as the wheel drops, if you wanted to modify the front body lip, suspect a 28" rear tire would be possible.
The issue for the front tires is mainly contact with the wheel well when turned to lock.
So if you wanted the same tires front and rear, the issue would be how large you could go without wearing through the wheel well.
A secondary issue would be whether the ABS would become confused by the faster turning rears, And then there is the TIPM.