RE: More Power
Late '80's I scratch built a generic shoulder wing model for a Black Widow, pretty much ugly stick layout but more normal surfaces. The fuse was 1/16" with light built up formers, the wing used 1/32" D-box and cap strips, nice 6lb C-grain tail surfaces etc., all covered in one of those light synthetic fabrics and a clear coat over the oil-prone areas. The arifoil came from a Top Flite combat kitten, a generic symmetrical section. Wing was maybe 6x32". And quite nontraditional (this is for you CP) was the firewall, just big enough for the BW backplate. All up weight 12-13oz. On a Black Widow and 5x3 prop it hauled butt. Of course top speed is capped by prop pitch and that wing, but it was brisk, accelerated hard off the hand launch, had huge vertical, rolled like crazy, and glided like a feather. I even managed to thermal it a couple of times.
The point of this being that it was such a huge leap in performance compared to the Mini Falcon / Mini Fledgling / typical half-A of the day, it kind of took me by surprise. And it was all about weight. Above that it had good proportions and sealed hinge gaps. A guy from Toronto who built a lot of small models, .020 - .049 RC sport stuff, took it for one flight and said "that's the best flying 1/2A airplane I have ever seen".
I have an old Mini Falcon fuselage downstairs built by a buddy - without engine, gear, or wing it weighs more than most of my current 1/2A models.
These days with every day radio gear so small and light there is far less excuse for any 1/2A model to have to fly like a brick. Well, except maybe for the status quo of the readily accessible decent performance 1/2A engine market which is kinda the OP's problem, ain't it. Pretty much the opposite of 20 years back. Mind you if you're willing to lay down $160.00 - $225.00 for a high end engine, you can go like a buggered ape any time you want. If you are happy without throttle, don't mind pressure fuel systems, or propping your engine for 25k+ that is.