The plate is prety easy to make if you own a small piece of 1/4 inch plate, a hacksaw, a drill and some files. I presume The MP ecu doesn't somehow compensate and take away power once the MIL illuminates, but I don't have any proof of that. As to the why, it gets some heat out of the intake, and may allow a bit more power at part throttle- EGR is not requested at full throttle. It will allow a bit more room around the engine, and your intake and throttle body will stay cleaner over time, but I don't think it really makes a ton of difference as far as power delivery goes.