I'm not sure if this plane was designed for ROG takeoffs. Have you tried a hand launch? It doesn't take as much power to fly the plane once it's up in the air.

Find a space where you have a long, straight, unobstructed stretch where you can land the airplane. Have a helper start running with the airplane at one end, and instruct them to release the plane ONLY if it feels like it will fly. Do not THROW the plane. Running until it feels like it's flying on its own ensures that the airplane will have enough airspeed to glide safely to the ground if there isn't enough power.

Keep the airplane flying straight and level. Do not try and turn or climb until you are sure there is enough power for the airplane to fly on its own. Do not add more and more up elevator to force it to climb; the airplane will stall, crash, and break! If there isn't enough power, the airplane will slowly glide to the ground. Otherwise, the airplane will fly level and begin to gain a little speed. Next, attempt to gain altitude. Again, don't yank in more and more up elevator; the plane will stall and crash!!! Full power, and a little up elevator will tell you if the plane can climb. Once you've climbed to a safe altitude, you can start making turns.

The biggest mistakes people make in flying slow floaters are that they don't launch them properly, and they try to maneuver them in too small a space.

For a speed 400, a 3:1 gearbox, 10x7 APC electric prop, and 8 cells is going to give you the best thrust for a slow flier. The same setup will work on a 7-cell battery, if you increase the propeller size to 11x7, IMHO. Use the 6V Speed 400 motors, not the 7.2V.