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As a general opinion, the f30 tire choices from the dealer suck. I believe the Goodyears on my sportline are 340 treadwear so changing them even in the same size to something good in the 60 area (pirelli corsa system, michelin sport cup) would fix the problem with the cars stock power although the car comes with too much body roll to begin with and this would make it worse because suspension needs to match the tires and weight of the car.
In general 225x45x18 to 245x40x18 (to keep a similar height) would be about 7% wider and less than 1% longer in contact patch but square area of tire doesn't mean traction nearly as much as compound or even tire pressure (non runflats could be more than 7% wider by just being under inflated)
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