Just wondering when do I replace my transmission

"Run better" can mean a hundred different things. If your drive train is still under warranty the dealer will repair or replace whatever is necessary to return the car to proper operation. Warranty doesn't cover modifying anything. In fact, many modifications that attempt to improve performance result in sacrifices in other areas like reliability, decreased fuel mileage, or increased emissions, and those will void most warranties. The automotive world is extremely competitive and if a manufacturer could add a few horsepower or increase fuel mileage without compromising other aspects of the car, you can be sure they would have done it.