The next "usual suspect" would be the exaggerated bubble canopy.
There was a sport plane around here with somewhat of an Extra look, but slab sided.
The canopy was flat across the fuselage, although looking normal in the side view.
The plane got passed around thru a few fliers, as it was a very poor flier. Difficult to land,until one guy put strakes on the sides of the canopy at the top edge. About 1/4" square and not very long. This made all the difference in the world.
The cowl bulges are quite noticeable on the GP CAP 232 I have, and along with the bubble canopy could be disturbing airflow at high alphas.
So now there's two things to change... the cowl and canopy.
Yesterday while talking to Tony, I was observing his sport plane, the Top Flite Contender has a layout similar to the CAP; low wing, high horizontal. No ballooning problems.. and no fat cowl or exaggerated canopy.