If you're comparing two identical wheels, then yes. 21s will perform worst than 20s, as 21s will weigh more (more unsprung weight). The 2 main reasons why wheels are getting bigger are: (1) to accommodate larger brakes (2) stiffer side walls for better handling.
I don't think you have brakes that need 21s over 20s. With 21s, you'll have less rubber protecting your wheels and your behind. If the cost of tires is not an issue and you just want 21s for the better look, go for it.
