MPH is a much better show of horsepower, but:
If its a mild combo, .1 isn't that bad. Obviously the better part is only going to be able to improve power if the original part is a restriction. Alternatively, if there's a restriction somewhere else (carb or heads for example) putting on a better intake won't help much. If you're running an edelbrock carb, there's your restriction. Out of curiosity which cast intake are you running?
On the 318, I'm seeing similar results with the same Street Dominator intake. I believe the issue is that the 318 doesn't make power high enough to take advantage of the single plane. Every single vs dual intake test in history shows that the dual is better down low and the single is better up top. Problem is, the 318 never goes up top! So you and I are taking the torque and power hit down low, but not revving the engine high enough to see any potential gains up top. My 318 is stock other than the intake, headers, and the Summit 6901 cam, and only makes power to 4950 RPM. It can't even get to where the HP and TQ curves cross at 5252! With the low compression and crappy 318 heads, it probably won't no matter what I do.
I'm absolutely certain my car would pick up with a dual plane over the single plane. I'm spraying the car so I am reluctant to put nitrous through a dual plane, however as soon as the weather improves I am going to try it. If I can borrow or buy a reasonably priced LD4b in that time, I'll do a back-to-back with the Action Plus. If not, its straight to Action Plus.