I received the correct centering rings from TireRack yesterday! They sent me 75mm to 65.1mm, but all (modern) Mercedes use 66.6mm hubs.
I mounted all 4 wheels and drove around the block. Front wheels lock to lock seem to work fine. Rears also didn't rub going around the block. Looking at the compression coming down my drive way gave me a couple concerns in the event of full compression... but I'll explain with the pictures:
First off, these wheels look great. They clear the front brakes nicely and have a very clean design. I wouldn't use them on anything but an AMG just because the spokes are too thin and they need the big AMG brakes to fill the gap.
Here are the front wheels mounted. I needed random wood in my garage to get the front high enough to use my jack under the Reverse Logic jack pad adapter after lowering my car on Eibach springs. Specs: 18x8 ET35 Enkei PF01, 245/35/18 Hankook Z214
On to the rear wheels. Enkei PF01 18x9, Hankook Z214 275/35/18. With the car sitting normally, the wheels look like they poke out by about 1mm.
But when the suspension compresses, they tuck snugly into the wheel well. This was taken coming down my driveway at an angle.
Close up... the bumper stick-on flaps will have to come out. They go a few mm inward and hit the tire when the suspension compresses. Was never sure if I liked these flaps anyway. They were there when I bought the car.
View from the back. The wide rubber looks good! Note the rocks and debris stuck on the tires. Driving around the block drew stares from the neighbors because of the noise of rocks being thrown around by the super sticky soft/verysoft compound tires.
On another note, these wheels are incredibly light. The rears, even being half an inch wider and 275 vs 255 tires, feel MUCH lighter than the stock P30 wheels. They are listed as around 19lbs a wheel. I believe the stock P30 wheels are close to 27-28lbs and normal SLK55 16 spokes are around 26lbs.
I am considering rolling the rear fenders slightly just to be sure I have no issues at full compression. The first autocross event is on Sunday and I'll be sure to report how it goes!