Allow for use of personally pre-tuned vehicles in spec events as long as they are evenly matched to other cars in the car class. That way if you have a car that meets the requirements, you dont have to go through the process of re-tuning your car to suit your driving style.
The spec events should either have everyone in the exact same car or in cars with the exact same rating so that it is as evenly matched as possible. If your car has a lower rating, you should be allowed to CHOOSE your upgrades to ensure it is at the same level as all other cars.