8" balls! Wow, that sound huge! But fear not -- the official site indicates that the balls are 8" in circumference, not an 8" diameter. That would be pretty close to the size of a tennis ball -- nominally, a tennis ball has a circumference of ~8.26"
Did you get a sense of the speed ranges for this device? Will it work for a relatively small area (short distance feed)? I noticed that they also have a cheaper machine that uses smaller balls. It holds 26 balls that are 5.125 inches around. Smaller ball might be a more challenging target.