One thing that may help is what is the intended use of your car? By that I mean where will it spend the majority of its time? If this is an occasional track car, dedicated track car, etc? Set your build up for where it will spend the majority of its time. If your going to cruise this 80-90% of the time then use 2.5" duals. If this is going to be a 50-100% track car then use the 3" as it will give you room to grow. Also, you can use 3" to the muffler and 2.5" after, several guys have done that. With that, any car ran at the track needs to follow the track rules, so you should look those up for what supporting mods you will need to run at the speed you are attempting to run.
