I have raced the Spec B car at a regional and it was very good indeed, i came up with the Spec B car as you find that you get torque steer with spec A as a lot of the weight is over one side.
The spec B car is very well balanced, and fly's extremely well, the rear gearbox is mounted upside down so there is no need to adjust motors etc, with both cars there is a need for a little more weight towards the rear, the spec B car corners like a 4wd and can carry a lot higher cornering speed than a normal 2wd.
Things i need to work on are, better slipper adjustment: As with any mid car when you go on a low grip area it shows up a lot more than a normal 2wd, i would guess the X6 guys would of noticed this.
Alternative front end: maybe a option to use the B4 still looking into this, but this would then make the front and rear ends AE.
Tub chassis:
with some different machining of the S4 blank chassis this could be used, this would elimante the need for a top deck.