See below. Would such a device increase exhaust flow? Basically it's a funnel forcing air through the end of the exhaust, causing an increase in flow. Might be too much, dunno. If it is too much you could put a gate valve on it thats controlled by speed, throttle positon, MAF signals, etc. Would/Could it be beneficial?  green is typical exhaust flow, blue is the "funneled" air flow due to speed. Outside the box again, but I thought of it a few weeks ago and raised a few eyebrows with ford guys at work.