I bought the other RB-S from Windsor and I have exactly the same problem, both rbs's are already set up as forward only, If you cruise around at half throttle it will go forward but as soon as you mash down on the throttle it sends the motor from forward to reverse with no pause or delay inbetween.
Reverse isn't a gentle or half throttle reverse, no, its full on reverse at top speed, it even brakes traction and does tyre spins while its in reverse.
I'm running the x12 on an 09 xray like Mark but of the 21.5 flavour, the wires are original and so far I now have multiple stripped / damaged gears and other bits in the drivetrain, the x12 is also buggered / cogging like crazy now but I supose instantly going from half throttle forward to full throttle reverse would tend to do that.
Very interesting to note is that if you completely turn off the Turbo function, as well as having a slow car, the RB-S behaves normally and will NOT go into full reverse.
Speedy, motor and stripped spur have now been swapped out for a brand new x12, a near new Orca and a new spur, everything runs great now.
To me it appears as though instead of hitting turbo the RB-S is going into reverse, my guess is either;
Turbo kicks in with a massive timing advance that is taking the final timing way over the 60 degrees mark and causing the motor to spin backwards ,as in 60 degrees is the half way mark of 120 degrees which is the number of degrees between phases of a BL motor. (360 / 3 = 120)
There is a firmware error where the reverse function have been missmapped or overwritten into the turbo function memory address range. (sorry I'm showing my decades worth of expirience in microcontrollers)