Desert Fox,
Several points. I have a fair bit of experience with the mods (It was guys in my Pl that had Al Janes make the mods to their vests). What they did was have the normal mag pouches removed and had a PALS panel put in their place. They had pals put on the back of the issue mag pouches so they could return their vest to a state where it at least looked normal. They mostly used Dropzone rattle resistor mag pouches.
There are pro's and con's to this mod. On the plus side it increases your mag capacity however I think that the cons out weigh the pros. I know a couple guys had some issues with the mesh starting to rip (the entire front of the vest is mesh, by adding the pouches you are doubling the weight on the mesh), I also know some guys ended up not liking it as it made accessing the lower pouches on the vest very dificult. That being said a lot of guys that did the mod liked it and it worked for them, to each his own I guess.
I decided to go with the non-permanent mod (as shown early in this thread) for my vest for several reasons. Having to turn in a modified vest is NOT a concern I have, and I question people that are concerned about this. If you are deploying and using the vest you are putting your life in the hands of your equipment. I needs to work and anything that can be done to make it work should be done. If you end having to pay some money 10 years down the road so be it, at least you will be there down the road to pay it. The main reasons that I didnt make the permanent mod that you are talking is because a) I dont think it is a very good solution and more importantly b) it is quite costly. I chose to spend my money on a complete non issue rig as opposed to spending the money on the mod. Again, personal preference.
As for mag closure, again personal preference. I personally prefer just velcro closure. I find it faster and more streamlined. But, and theres always a but, it depends where the pouch is mounted and how it will be used. For the mod I made to my vest I chose pouches with fastex closure because the pouches are mounted in a position which makes them much more prone to accidentally opening. If you need the security get the fastex, if they are mounted on your chest and you need the speed go with velcro.
Hopefully this helps, merry Christmas.