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The wheel specifications look correct as far as I can tell.
When you go up on the offset, the wheel will be moved more in wards. Since the 21" wheels have a +30 offset, I don't think you'll have any clearance issues running +30 on 20" wheels. But the effect will be that the wheels will look more "tucked in", and less flush with the sides of the car. You can of course fix this with 10mm spacers on the back and 5mm spacers on the front.
I myself am not a fan of spacers, but if you are not too crazy with your driving, they'll be fine. Personally, I'd go with the 9Jx20 front rim, which already has a 25 offset, which is nearly an exact match for stock.
Mercedes SLR McLaren 722 S Is Extremely Rare Example Modified by McLaren
Slideshow: A one-of-one U.S.-spec Mercedes-Benz SLR McLaren Roadster became even rarer after a factory-backed transformation at McLaren's headquarters.
