Hi Gordon.

I also have the E-flite Extra 260 with the Showstopper, w/TP 730 ( a 900 is better ) and CC 25 amp. The reason why you don't have the same thrust in reverse as forward is because your motor is taking all that extra torque from the down movement in the prop making it slow down ( flat props are like air brakes ). You need to add more power to the motor in the way down than up and more pitch also to keep it from over amping your set up. You didnt said what ESC you have but one with more amp handling is better. If you have a metter use it to set it up and check for the amperage. My point is that your set up for regular fly is not the same for the reverse one. Hope this help any other question feel free to ask.

Fernando