From what I gather, KPM make the system but outsource the mufflers. I think the design of the system is done by KPM.
Hi Josh. Mate if you're worried about upsetting the neighbours, run with the 2.5. They have a 'quiet' or 'medium' option. The quiet one will still have a big of a rumble at idle and will bark under load but will be much subtler than the medium. Maccas, from what I understand, has the medium 3 inch, with the loud being a track only unit. Read the flow rates as increasing from quiet to loud. If you're keeping the rest of the car stock or nearly stock then any size will be ok. But if you plan kids like cams then you'll need the increased flow of a 3 inch. The idea is that a 2.5 will retain heat in the gasses better so they can move faster through the system and 'scavenge' of actually help with pulling gasses out of the cats. Merge pipes help with this also, much the same way that well designed headers will extract gasses from the exhaust ports in the motor and prevent any back flow. There is some evidence that a 3 inch will not maintain the same heat in the exhaust gasses from a stock or nearly stock motor as the rate of flow from the motor is not sufficient.
But the difference in day to day driving would be minimal so your best option is to go with the sound you like best.