I did a lot of racing and could never really notice much difference in air filter brands. I could however notice a difference between a clean and a dirty filter. For $10 I am a firm believer in replacing the air filter element with normal maintenance, it is one of the most neglected maintenance items.
The placement of the motors in these things cause it to run in very dusty conditions, a lot of dust comes up between the seats. I am thinking on adding a snorkel just to get cleaner air to the motor.
Don't feed the bears no matter what they tell you!