I've been playing with an HDR1000 monitor for a while and have not seen issues with the sun blowing out clouds. In fact, when I toggle HDR on/off highlights are much less blown out. In external view I can have a super bright specular reflection from the sun right next to the TBM logo on the 930 and still clearly read the logo. In non-HDR half the airplane would be a fuzzy white mess.
The only time I've seen the image blown out is when I'm inside the cockpit with the interior taking up a significant portion of the screen and it's bright outside. In this case everything out the windshield gets semi-blown out. But this is a result of the game's automatic brightness adaptation and not HDR; the same effect is there with non-HDR mode.