I bought the standard kit, used the supplied motor and prop, two standard servos, standard radio, and 8-600mAh NiCd. The weight came right around 23 oz. and the C.G. was dead on. My first two flights (a day apart to allow for repairs) has a total combined flight time around 4 seconds. The plane just dove into the ground and mangled the nose and canards.
Since then, I've converted it to a Zagi-style flying wing and learned quite a bit about making this bird flyable. For making the standard (or deluxe) kit flyable without extra parts or a lot of effort, these tips might come in handy:
-The trim of the ailerons is critical. The directions don't mention this, but the ailerons are at neutral trim when the bottom edge lines up with the bottom of the wing. To accomplish this, turn on the radio and set it to neutral; put a straight edge (i.e. a ruler) along the bottom of the wing about 1-2 inches forward of the trailing edge; adjust the aileron so it is parallel to the ruler and tighten it.
-Make sure the center of gravity is at the right spot. Measure it according to the instructions and move the battery around until it matches.
-Attach the control horns and hardwood to the ailerons such that the shape of the hinge is preserved. If most of the aileron has the appropriate 45ish degree slope and the hardwood is stick out beyond that, your hinge won't work.
-It's a nice touch to round a few corners with fine sandpaper. The leading edge of the wing, the top of the trailing edge of the ailerons, and the leading edge of the canards could all use a little touching up.
-Check the balance of the plane on its longitudinal axis. You might find it helpful to run the antenna along the right wing to achieve better balance.
-It may be worthwhile to run some strapping tape along the nose and canards before your first flight. I don't have a nose to test it on as mine is in many pieces, but I think that might be beneficial.

I've only flown my modified Firebat once, but it had plenty lift and handled nicely. I can attempt to describe my inexpensive conversion process if anyone's interested. It's specs are std. motor and prop, 8-600mAh NiCd, 2 HS-81 servos, Hitec 555 micro receiver, JES 110 ESC, GP Slinger elevon mixer, a foam-rubber mouse pad (yes, like the one you're using now) and typical building materials.