It sounds like you need a prop that will let the engine rev a little more, if it will not speed up from 90 to 100% throttle opening it is because it simply does not have enough power to rev any faster. That is not a flat spot. A flat spot is when an engine is accelerating and the torque curve flattens out briefly or even decreases some, then resumes building torque. My brothers race car engine has a flat spot at 4,400 rpm where it drops 19.6 foot lbs. of torque with a corresponding 100 degree drop in exhaust temp. Obviously a carburetor problem. Couldn't be in a worse place because it's right where he hits the straight. I believe a slightly less load would cure your TT.