My 2 cents
With engine mods I have :-

Custom cold air intakes
Modification of the airflow system
3" free flow true twin system 75 Db @ 30 meters to meet FAI regulations
Removed two of the cats
100 octane custom built ECU (Powerchip) Shell V Racing fuel but will be moving to E85 for the road.

I have removed the standard seats and put in a single Sparco Kevlar seat but run a full cage.

Brakes Pads suspension wheels tyres are all upgraded but it outruns my SRT XF until around 70/80MPH I haven't had them down the drags together but my but says there is nothing in it.