I'm not sure what more the Spy can be made to do. He shuts off power in the Power Plant, establishes a team-wide relay in the Radar Dome, sabotages the ore dump in the Refinery, reveals enemy vehicle locations in the War Factory, does the same for Barracks(?), and disables repairs in the Construction Yard.
The only idea that comes to mind is changing the Spy's effect on the Barracks/War Factory and jacking up the prices for the Soviets. A successful infiltration and the Soviets are forced to buy things at 25% markup for the next 30 seconds or instead wait it out and let the prices return to normal while they hunt down the Spy. Other than that, the Spy does a phenomenal job at creating opportunities for his team and causing confusion and stress for the enemy.