For me, the 501X was 'cool' without the one way - and 'AWESOME' with. Totally transformed it. You don't NEED a one way at all, but it does liven the car up and make it more agressive, which needs a smoother / different driving style.
The only time I ever put a diff in the front of the 501X was in Finland, and I went from getting laps not far off the top drivers, to actually finishing races without crashing instead. Make of that what you will
