For those who were curious on how the testing turned out, here is the report:
I used the Esstac Bungee Retention pouch and the BDS Six-Packer. The Esstac was mounted on the very far left side of a Blackhawk Rhodesian rig (I used the partial 3 strip tall PALS layer to mount it). A Maxpedition RolyPoly was mounted on the bottom of the Esstac single-mag pouch. The BDS six-mag shingle was mounted dead center on my chest with nothing attached to it. Also I used a mag with Magpul in the Esstac pouch and normal GI mags (no magpuls) in the BDS pouch. I probably should have reversed that.
Assuming perfect runs (no mag fumbling, bungee malfunctions or anything else), the bungee retention was about 0.2 sec faster on average (counting only perfect runs). If you include all runs, the flap is maybe 0.1-0.2sec faster.
Note that I only did a 3-4 perfect runs and maybe 10 runs each total, so this is hardly a statistically valid number. The problem with the bungee is that when it slides off just like Blackhawk 6 described, it is a lightning fast mag change. If the bungee catches on something, slides off but catches the corners of the mag, or you trap the bungee with your hand - the mag change goes awry quickly and is slow to recover. If you are going to use it, slow practice to build up correct muscle memory is probably worthwhile. The grippy rubber material of the Ranger plate also made sliding the bungee off a little more difficult than a smooth GI floorplate would have.
On the BDS 6-shingle, the mags went in very tightly. You can definitely NOT use older style magpuls with these. As a result, the first mag out of a dual mag pouch was always slower than the second mag (and slower than a perfect bungee run). The most common flub here (I was using regular floorplates with no type of magpul) was grabbing the mag and having my hand slip off it as friction was too great. However that is a pretty easy flub to recover from. The BDS lids are tall enough to accomodate the Ranger plates and I would definitely recommend that or paracord for at least the first mag in every pouch. The second mag came right out every time.
Both pouches did a good job of retention and keeping assorted crud out of the mags. The Esstac pouch features "adjustable depth bungee retention" but there isn't all that much adjustment room as it is a single piece of cloth velcroed on both sides (so if you lift one side of the lid up, you also lift the depth the mag can be stowed up). However there was enough adjustment room to make using them with Ranger plates feasible.
I think I am going to continue to use both products in the same location. I plan on making the Esstac pouch my "fast access" mag and using a plain floorplate there. I will use Magpuls on the BDS pouch and see if the times on both improve.
I really liked the RolyPoly hanging off the Esstac for a quick mag change. You just drop the empty in the bag and as you come forward you grab the mag in the Esstac pouch. I think the a bungee retention pouch is probably faster mounted towards the strong side outboard where the natural motion of your hand during reload works with you. Mounted inboard and up front on your chest, it may be a bit slower.