To reduce the flapping that can occur with strong winds, there is a commercial product called "Air Wedge" that is placed between the slide roof and the slide topper. (See
www.awningairwedge.com) for details about this product.
Instead of purchasing a ready-made commercial wedge, you can fashion one of your own. Some members insert beach balls or something similar .... I have made a wedge from rolled up carpet underlay. My wedge has a looped rope through it so I can easily remove it without climbing up on a step stool or ladder.
In some instances where the wind is severe and the slide topper is subjected to too much stress, members have had to retract the slide to reduce the noise and the possibility of tearing.
To clear the rain off, you can always retract the slide and the water cascades off the topper and we have never had any water coming into the interior of the RV. The home-made wedges also help to prevent the rain water from pooling on the top.
I'm not sure if a topper would help reduce the temperature inside the RV on hot days. We don't spend much time in the "heat" so no first-hand, real-life experience to make any comment about their effectiveness in keeping the unit cooler.
If anything, I would think that a slide-topper would help reduce the likelihood of water penetration into the slide out.
All things considered, I would recommend a slide topper.