20 inch wheels, brand new - 8mm Pirelli Tyres..

21 inch wheels, 6.5mm (Part Worn) Continental Tyres..

6.5mm Continental tyre weight..

I know that the 8mm new tyre = 13.1kg
Which means....
20" OEM Wheel and Tyre = 25.8kg per corner
21" OEM Wheel and Tyre = 29kg per corner
Means you add around 13kg of additional weight with the 21" wheels, theoretically the car with the 20" wheels will accelerate marginally quicker.