'Opening up' the original exhaust will achieve nothing but noise and will open a can of worms as far as the fueling is concerned.
While, if the original map is retained, the ecu will try to trim around any changes in the closed loop area it will be crude and far from ideal even if it doesn't throw the signal from the lambda sensor out of range and cause a 'Service' warning to be shown.
The original exhaust works surprisingly well. It's biggest negative is its weight! By far the best bet is to choose an aftermarket pipe, leave the dB killer in and map appropriately.