I jump MFF, and I would not jump an open top KYWI.
I like Esstac KYWIs. I'm still currently testing out some Esstac triple mag panels for my primary rig. But I know for a fact you should not jump them. You can maybe get away with the Esstac Tall pouches during a jump. Esstacs are great for flat range training, weekend warriors, and niche CQB situations, but think about it. You pull the mag out with a small amount of force - with your hand - so your mags are going on a field trip if some external object hits you at the right angle hard enough, if you have a hard parachute opening, etc... I had an opening one time that was so hard that my breakaway NODs mount fulfilled its purpose. If I was running KYWIs/TACOs/Scorpions, I would have landed with maybe 1 or 2 mags to fight with.
I don't think any mag pouch setup is perfect. I've been at this for almost a decade and am still on an ongoing quest to find the best combination of the speed and retention of a bungee/tab mag pouch and a flapped pouch, while ditching the extra weight and bulk of kydex inserts