ORIGINAL: Mr67Stang
You did not mention which Extra 300 you have. Since this is the beginners forum I would guess you probably have the Park Flyers Extra 300 that advertizes, "Hover at 1/2 throttle"??? If it is I hope you have some experience prior to that plane. What will likely happen if you set your ailerons up the way you describe will be an adverse downward pitch of the nose. I know that does not make sence however, all but one of my models will do this when I apply Flapperons without mixxing it with up elevator. Since you do not have the ability to mix I would encourage you to learn to land it hot as opossed to adjusting the ailerons and elevator. Now I have no idea of your previous experience so forgive me if I presume to much.
Best of luck with whatever you decide.
I used to have that problem until I started playing with the CG. On all my 3D planes, I have the CG back just to the point that when I apply flapperons, the plane almost comes to a standstill. Most of the time, this is just behind the recommended CG (also helps in a hover).
If you could move your CG back just a little at a time, I would be willing to bet that the plane would fly much slower. Being that this post is in the beginners forum, I would suggest staying within the recommended limits until you are comfortable with the change. I like flapperons, but I really only need them on a plane outfitted with a 2 stroke engine, and I never use them for landing. Flapperons take away the effectiveness of the ailerons.