50mph isn't really that slow. If you think of sitting on the side of a highway with cars doing 55 mph right in front of you they seem like they are hauling. Most 3D and Funfly type planes don't want an extreme variation in speed they just want to go at a moderate speed with extreme amounts of thrust. Watch someone with a funfly plane with a 3 or 4 pitch fly around they move almost the same speed no matter if they are at half or full throttle. The thrust program is just a general guide, in practice you'll find the 18x6 just fine.