3kPa or 12 inch of water is good to keep as the limit.
Too high backpressure will hurt the combustion. Too high back pressure will reduce charge air into the cylinders because of the restriction to get the exhaust out. This will affect the air/fuel ratio with thermal overload as the result.
The biggest impact from the SCR and oxicat device will be from its cross section and length. Best result is achieved when the device is made as wide and as short as possible while still retaining a good efficiency on the device.