I know this is a really old post, but I figured it'd be better to weigh in on this one/revive it than start a new one and clog up the system. Mods, let me know if I'm out of line. I've tried out the Esstac KYWI Kydex inserts on ex in Canada and the US extensively in the past few months, and I find they're an excellent addition to the issue tac vest mag pouches. I can't speak to the quality of other kydex inserts such as the Haley MP2 or airsoft variants, but I imagine they function similarly.
Regarding installation, the KYWI's velcro seems to be unnecessary. Just slide in a midlength 5.56 KYWI insert and it'll stay in the TV mag pouch, albeit maybe a little tighter than the manufacturer intended. Another solution is to stuff some rags in the bottom of the pouch, raising up the kydex insert a little and leaving more mag protruding from the top of the pouch to grab onto.
If you like to run your mag pouches open, then you'll probably find the KYWIs very useful. I've always found that tucking the mag pouch flap - buckle and all - into the pouch can make re-indexing mags a PITA and next to impossible if the buckle decides to block the path of mag going back into the pouch. I know some guys like to tape their mag pouch flaps/buckles up to the TV shoulder straps, and this can work; however I prefer removing the male buckle from the top flap (leaving it clipped into the female buckle on the mag pouch front) and tucking the mag pouch flap and (now buckle-less) strap in behind the KYWI insert. This may take some finagling but once you've got it oriented right it'll stay nicely tucked in place behind the KYWI and won't get in the way of re-indexing mags.
If you like to run your mag pouches buckled closed, then I still recommend the KYWIs. They provide an extra assurance that your mag will stay in place, and if you don't have the time to re-buckle your mag pouches after re-indexing an empty mag, then you won't have to worry as much about said mag jumping out whilst on the move. The kydex inserts grip the mag securely and I haven't noticed any loosening through Aussie peelbacks, section attacks, jungle lanes, urban ops or patrolling.
This of course only applies to those of us still using the issued TV for one reason or the other. If I worked out West, I imagine this wouldn't be an issue and I'd be using some sort of fancy commercial mag pouches. But for now, the KYWI inserts (plus an HSGI Taco mounted on the front MOLLE of the TV) will more than do. It's not as fast as my TACO pouch for emergency reloads, but it's still significantly smoother than it was before. Hope this helps and please take my review with a grain of salt as this only reflects my personal experience.
Edit: here's a link to where I bought mine. https://www.esstac.com/accessories