We've been experimenting with single-wheeled shooters such as this, and today we finally tested with multiple balls.
Here's a picture of the setup we used, with about 1" of compression.
The wheels were wrapped with a self vulcanizing tape (a fairly sticky rubber finish). We saw pretty large variances from ball to ball. One ball in particular shot consistently higher by a couple of feet. We found that the coefficient of friction of one ball on the rubber wheel surface was much higher (like a factor of 2!), but we couldn't discern any other difference between the balls. Very strange. This particular ball came from andymark in the same box as a 'slicker' ball, but I don't know if it's from the same batch.
I'd suggest that if you make a wheeled shooter you test with as many game pieces as you can find!