The slow backwards driving was a result from the earlier skipping which you fixed, so that shouldnt be a problem anymore, all i can think of is that you have a defective 2.6 motor. Contact hobbico (they have great customer service) and tell them you dilemma. They will gladly replace you motor.
But before you even do that ride around your microsizer and 2.2 if the 2.2 seems to start off the line fast it begins to go slow. Try to test both of these motors and you may notice that the 2.6 will quickly gain its speed.