Running without cats...: rear lambdas have to be disconnected+ modification on the ECU.
Did have normal milltek back box for just few thousands of miles had to replace them because the filling (rock wool) did come out !!!! the back of the car was cover of it, i had the only RS in the country with an hairy ass!!
and the noise was awfull at the end, had to drive the car with hear plugs on !!
Had some special ones fitted with heat resistant material in it, now it is not too loud inside the car.
Get ready gentlemen, it is pretty loud outside especially under accelerations........Yeah!!!

Porsche 996 Turbo S Atlas Grey.
RS4 B7 Avant Sprint Blue. Sold.
RS4 B5 Avus Silver with MRC magic . Sold (Best car ever )
S4 B6 Avant Nogaro blue. Sold
A6 Avant Silver . 2.5 TDI .Sold