no offence mate, but your miss information is way worse. cold air is not thicker at all. air density decreases with an increase of altitude, as does air pressure, nothing to do with air temp, although o2 is more present in air with a higher density and lower temp.
back on topic, im keen on a x force cat too, does anyone know the length of it? i know the stock cat for an r32 gtst is 310mm, so hoping the xforce will be very similar.