With an X pipe, the exhaust mixes the gas from different banks of the engine block, producing even gas flow to both pipes at a faster rate (as suppose to pulses of gas) Straight pipes will output gas unevenly due to the firing order of the cylinder blocks. So using an x pipe would be slightly more efficient.

With the removal of the resonators, the exhaust will naturally be louder. Using an x pipe will quiet down the exhuast system and produce a much smoother tone in comparison to just running straight pipes.