What I don't understand is why neither Cargraphic or Ruf powerkits - any level - have a replacement plenum in them

For example the 650bhp Ruf package has: Fine tuned engine electronics, sports air filter, conversion of exhaust with 4 round 76 mm end tips and sports catalytic converters, 2 turbo chargers in exchange, titanium connecting rods, 3.8 litre cylinder capacity, transmission oil cooler and stronger limited slip differential for 6 speed manual transmission, stronger clutch.