Your car now looks like it is a 4x4 ;) because of the reduced sidewall and increased "metal" visibility. As you have figured out, the 245/40 tires on your 18s are the same outside diameter as the OEM wheels.
Going to 245/45 will reduce the gap but that is because the outside diameter is about 1" more. The result is 1/2" gap reduction and 1/2" more ground clearance. Because of the extra diameter, your speedometer and odometer will also be slightly off. Depending on the rim offset, you may also have some rubbing problems when you do full turns and reverse. You may also rub the top of the wheel well liner if you are on the freeway going fast enough and hitting a big dip. A nice thing about the 245/45 size is the extra sidewall to give you a more soft ride.
With all that said, your best bet is to go with a mild drop with aftermarket springs and shocks. This is the most economical way to do it. There are also coilovers but these will be more in the $1k+ price range. The L-Tuned setup is good if you can find it since the L-Tuned division is now defunct. If you are OK with the current ride quality, the shocks/springs will be fine. You will notice the car handles better and the car won't feel so floaty. You have already taken the biggest hit, in terms of ride quality, by going with the smaller performance sidewalls on the 245/40.