SLS Qatar

The stock spec are

Front:
263/35 ZR19

Rear:
295/30 ZR20


Would I get a better grip and acceleration If I replace the rear with 21 inch wheels and a wider offset?

Berde

Not necessarily, I would go with a softer compound high performance tire. Obviously, if the contact patch is bigger, you will benefit from more tire to the road. But, that does not translate into instant grip. It is going to depend on many factors, and tire compound is usually the big hitter here. As an example, race cars would use slicks for dry pavement, which stick like glue, but comes rain and switching to radials is a no brainer. No size change here, just tread design and rubber compound. In my opinion, fronts and rear sizes are substantial in the example you have given here. Just my 2 cents!