According to the theory I have read, removing the cowl will make the plane slightly more stable (in pitch and yaw, harder to knife-edge), as cowls and fuselage area ahead of the CG are de-stabilizing. It will usually be a little slower too, but maybe not noticeable. I say a little slower because few models have efficient cooling baffling in their cowls. If they did, then the difference would be much greater. I once read that a very large percentage of total drag (I wanna say 80%) of the full-scale Swift at cruise was engine cooling drag. That's why Roy LoPresti had such a ball cleaning it up. I could be remembering the percentage wrong, but I remember thinking "wow".