I think the fine filter would tend to be more restrictive than the coarse one in general. If the 56 has a milder cam there would be less of a difference in fuel consumption due to less spitback. Now that brings up an interesting point. Most of the spitback is at lower or mid rpms I think. We usually don't run WOT for a whole tank of fuel. Therefore if you ran the same test, but at mid throttle, there might be even more of a fuel savings with the stack.

Ernie