With EGR/DPF mod's and not too savage remap , say plus 20% bhp and torque , it will be so smooth to drive , without the hesitations present in a standard car. If you then drive it hard you'll use more fuel but my experience is that , yes , I do sometimes use that extra grunt because it's brilliant but driven normally , mine's used less fuel since the remap etc at Angel's Banbury. Running around locally on very short 2 to 10 mile journey's it does 35/6 mpg but motorway cruising 44 mpg. As said above MOT's aren't a concern because most remap places empty the dpf from the top surface then reweld it , so once refitted it looks as normal because the weld isn't seen. Not certain the mot stations will like a ''replacement pipe'' though as some have fitted ?