There will be more twisting on the transom as the force line of the propeller is lower. This may tend to lift the bow more as well. Will also cause more drag and spray. Will be good to exercise the engine though.
However, my brother had a longshaft 7.5 on a 2.8m humber SIB with no problems. One benefit is that when the boat flies the propeller stays immersed and you don't lose drive