The stress factor deserves some conversation here, because I think it's important to remember that you're not going to want to do a tac-reload while BG #3 is fast approaching from your flank. No, it is after additional threats have been scanned for, as SnB stated. Inherently this means that even though the adrenaline will still be pumping, you're under NO severe stress to get the reload done in quickly. That's the nature of the Tac Reload - slow and easy, get ready for something else.
When I do a Tac Reload, I can take my sweet time pulling the fresh mag and getting it into position, pulling the weapon in close, and then the only thing that happens fast is the drop and slamming in the new mag. Once mags have been changed, I can take my sweet time finding a place to put that partial mag. It's all about economy of motion, and the whole drill shouldn't be run at light speed, coz if speed is that much of a factor then might I suggest that just getting the new mag in is more important and you might just drop the partial anyways..
If you practice effeciently it then it doesn't degrade so badly as one might think. If you find yourself stressed and you drop the mag, well ok - no worries. You just completed a hybrid Tac-Com (Tactical/Combat) Reload; There's still rounds in the first mag but you don't worry about retention. Why? Well, situation dictates. You might have another threat to worry about and you don't need to add mag retention to the equation.
SnB - If I'm reading you right, the ONLY reason I don't agree with the KISS method is that once you've dropped the partial mag you have nothing except one round until you get that fresh mag inserted. It's only a matter of seconds, so it's all about whether or not that's important to the individual deciding on the method. However, if something happens during those few seconds you will discharge the one round and risk forgetting to reload. Sounds silly, but I've spent a lot of time with people under this kind of stress (training and real) and you forget the silliest, dumbest, most obvious things.
I'm not saying I've never DONE the KISS Tac Reload, I'm just saying that I trained myself to pull the fresh mag first
Like I said, Situation/Time will dictate!!