Should I go back to OEM size tires?

You will gain about 4% in RPM with the smaller size and the engine will produce a very small increase in horsepower by running faster. My guess is that you will have about 5% better performance and the drivetrain will benefit slightly. The cooling system will perform slightly better as well. If you might be operating at large throttle openings, I would use the smaller tires. If handling is the priority, then go with the larger tires.