Removing the cowl on an airplane with a rather wide fuselage will increase drag considerably, due all that additional flat plate area slamming into the slipstream, slowing the airplane and steepening the glide. Stability and controllability may also be hurt by the slower, more turbulent air flowing over the tail feathers. CG will come back a bit, causing a little higher pitch rate. I have noticed overall handling degradation in an airplane with a much smaller cowl in relation to its overall, size when only the top half of the cowl is removed. I would leave the cowl in place, except for experiments.