I've one of the Goldberg Cubs and tried flapperons. They only very slightly provide additional drag but reduce aileron authority by around 50% so unless you are good with the rudder, I'd add some aileron to rudder mix. I tried them and abandoned the effort and changed prop.

Go up one size in length and down one pitch and note the difference, it could be dramatic because it will provide much more braking action at idle speed and do far more to slow the model than flapperons. If you are running a 12x6 on a four stroker for example, go 13x5.