RE: AP Yello Jacket .15
I'm going to disagree with nearly everything Darren has said. I spend a lot of time running .15 engines in RC combat competition.
You don't need high nitro in these engines. I use 15%, and kick plenty of tail with my .15's.
Too much oil is almost as bad as not enough oil. You don't need more oil than a larger 2-stroke. I run Omega 15%, the same stuff you have now. Other guys run Powermaster or whatever their favorate flavor of fuel is. But you don't need excessive oil. Too much oil will actually reduce RPM and make the engine run hotter.
Now, why is the AP running weak? You actually have it nailed. The AP is known to be underpowered, and isn't very common in RC Combat because other .15 put out a lot more power with less work. But a couple of guys have really made them take off by doing exactly the mods you were talking about, open up the carb and exhaust and you will get a LOT more power out of that engine.