Have you tried indexing your plugs?
1. Take your spark plugs out and look at them:
2. Now at the bottom of the plug, there is that metal piece, and on that one side it's open. On
THAT side (in the image it would be the right side) mark the TOP of the plugs.
3. When you rescrew them into place make sure that the mark on top is
FACING the intake valves.
Now what this does is actually point the spark directly at the fuel entering the chamber, resulting in more efficient combustion process. To make better sense, imagine if the plug was facing the
OTHER way. It would then be actually blocking the spark from the fuel, not enough to be a problem, but when adjusted could add some needed horses, even i they're in the single-digit range.
I'd say this would give you.. roughly 5-8 horses, depending on your engine, and doesn't require anything but a sharpie and the engine itself.