One thing I've learned is that small changes can make better planes. For combat though, the first things are: a reliable engine that starts in one flip, holds a needle valve setting, somewhat affordable, and durable. A good pit crew. Depending on the event, the pit crew can be vital. In AMA Fast combat the pit crews probably win at least as many matches as the pilots by getting up first. Being able to get to, retrieve, clean up, fuel if needed, and change the prop in a minute or less. The plane isn't quite as important, but a good design in good trim will help a lot.
It's kind of a shame, but with F2D Fast the plane has lost a lot of innovation. In fact, designs have been optimized to such a degree that it's basically a one design event. Most flyers buy their planes ready to fly and there are only two basic designs- one with the stab hanging behind the wing on a small hinge, and one with a flipper elevator sewn to the back of the wing. The integral flipper seems to offer an advantage in smoother control while the stabilator version reacts faster with less handle movement.
But even today, small changes in the plane can make a difference. I've found through multiple successful design that roughly a 5% change in span, wing area, aspect ratio, or engine power can make a useful difference in performance. Take two planes with similar construction- extending a 36in. span to 38-39in. with a narrower chord will make the plane faster in maneuvers. Knocking an ounce off of any design will let it turn just enough tighter to give an advantage. The Gotcha 500 design I used for several years was a far out compromise with and advantage in turn radius(+10-25% wing area), wing span(10-20% span), and an engine that could turn any reasonable prop fast enough to go 110-115mph, which was all I could handle. The down side was that it was so lightly built it would barely last 20 flights before starting to get too flexible from fatigue.
The Nelson engine opened up new possibilities. I built a 550 sq.in., 56in. span design that flew very well. It was especially good against the ARF fast planes. They were restricted to about 46in. span and needed quite a wide chord. That design had plenty of speed, but tight turns slowed it down a lot due to the low aspect ratio.