Downforce at the front needs to be balanced with downforce at the back. Ford GT program engineers carefully designed and tested the front splitter, rear diffuser, and side skirts in wind tunnels to optimize the downforce and drag. Refer to the SAE papers that they wrote about their efforts and results, while maintaining the styling of the top of the car.
Anything you do will change that balance. More downforce will increase induced drag and reduce top speed. But hey, it’s your car, so if you really like the look of a larger splitter, go for it. Just realize that you’ll be making trade offs.