The Horton STC does state what the limitations are: Simply stated, it will meet or exceed the Cessna numbers and that you should go out and experiment with it to determine the actual numbers. So in otherwords, just use the Cessna book and you'll be happy that it does better.
From my experience the airplane will not stall with a Horton kit, just mush and gently hold the nose below critical angle of attack; my guess is the elevator is losing lift before the HORTON wing is, which is how tricical type airplanes should be.
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