To speak to the orignal thread question about wheels sizes and lowering cars "slightly" I wanted to share my experience.
I did thorough research into my combination of wheels, tires, and suspension setup to ensure I wouldn't have clearance issues (rubbing).
Well, after hearing people give testimony after testimony (even seeing that same chart above posted over and over again in different threads, even on different forums), I purposefully chose the least aggressive/least evasive setup to ensure I did not push the limits.
The choice: 18x8 ET45 w/225/40 Michelin PS2s + Neuspeed Sport Spring + Koni FSD shocks.
The result: RUBBING!
The reason: IMHO, people talk about rubbing without including the limits of suspension travel. So, if you have large load (several passengers or what have you) or if push the car into a corner at a spirited pace/level, you may or may not rub.
As for me...
The Neuspeed Sport springs advertise a 1" drop when in reality it is closer to 1.25", but this is really the norm for any aftermarket spring. Sure, I acknowledge, you can find spring that go lower or very rarely some that do not go as low.
What I did to cure the rub... used blue painters tape (color not important).
The painters tape is easy to apply and easy to remove with a non-aggressive adhesive.
Anyway, I put painters tape at all four corners (around the lip of the wheel well) and found my nearest round-a-bout to check clearance closer to the suspension travel limits.
What I found was the rubbing only occured in the front and at the top center, where the wheel well liner fastener is located. This is one of five fasteners that hold the wheel well liner in place.
The cure: Remove the top fastener and voila! No more rubbing.
So, my point?
Realize that no matter how certain someone is or how convincing they might sound on a particular mod subject, be careful in your choice and ask many questions to ensure all your bases are covered.
Back to the original question, wheels sizes for slightly lowered cars.
From my experience, I would not go any more than 18x8 ET45 if you plan to "slightly" lower your car. For reference, the stock detroits are 18x7.5 ET51, which means they are 12.35 mm or about 1/2" further tucked into the wheel well. Or, from the other perspective, the OZ wheels I chose are pushed out about 1/2" further (toward the fender lip) than the stock wheels. And 1/2" is a lot when it comes to wheel/suspension geometry in the wheel well. Those that are running 18x8.5 ET45 are another 6.35mm or 1/4" further out (50% more than my OZ wheels!). I think you see my point.
I know others will disagree and swear up and down that they don't rub, and likely they don't but that does not mean you won't.