Were you asking me or Erkki? Speaking only for me, I don't really like the small tip plates on the end of the rear wing, they will not provide anywhere near as "solid" of yaw stability as larger and higher aspect ratio fins. Taller, narrower fins on the outboard end of the rear wing can work (Ercoupe), but you give away any and all benefit of the prop blast. The prop blast on a large central rudder will be a big deal on a mono-wheel landing gear like what your rendering shows. Even if you have nosewheel steering forward of the main gear, once you raise the nosewheel off the ground during the takeoff roll, you'd be relying on rudder authority from then on. Having the rudder in the prop blast will increase that authority. If you are referring to the Spratt wing specifically as a free-wing with no direct control input to it, I don't like it at all. The Mignet system of a direct control linkage from the stick to the forward wing for pitch control, I can live with.