Not having one myself, and just reading testimonials, I have to say that many of the Firebat's problems are due to weight. IIRC, the plane typically comes out in the 25oz range. You might be able to get away with 25oz on Speed 400 power IF the plane has a huge wing area, and you geared the motor. In this case, you're trying to push jet-shaped plane with a jet-like wing loading around with a direct drive Speed 400.

The other major contributor to the plane's problems is the control surface setup. From what I've read, the canard and elevon setup is absolutely CRITICAL. Too much canard, and you create a lot of drag. Between the weight, the power, and the drag, the plane is not going to be able to accelerate or climb well.

Why Great Planes went with heavy Sub C cells, I don't know. As you've realized, you can substitute 2/3A NiCd or NiMH and save a few ounces while not sacrificing power or a whole lot of duration. The Zagi does well with a similar power system, so it makes sense that if you can get the Firebat down to <20oz on an 8-cell pack, it should fly fairly well.