I would suggest that at busy training airfields a small oval circuit would cause problems simply because of the amount of inexperienced traffic coming in and out all the time.
A bigger circuit obviously means more trafffic can get into the air and there would be less go arounds due to people getting too close to each other. Sure an oval would probably increase your reaction speed, but it's not much use if you can't get into the circuit to practice due to the traffic!!
Foz