I'll give my reasoning below, but I now think the wifi problem is roughly the result of the following two issues:
1) A (hopefully) software based buffering issue that prevents the laptop from efficiently handling marginal connections. This may be USB related as it happens with both internal and external wifi adapters.
2) Placement of the wireless antennas in the base (keyboard) of the device that make the laptop more susceptible from interference from objects such as solid tables, legs/torso, hands, etc.
Today, I took the USB wireless-G adapter and attached it to a Samsung laptop with inSSIDer. The signal strength is identical to the yoga, but there is no stuttering issues. But I then played around with the Samsung laptop. I turned it upside down and rested the screen on my lap with the keyboard in the air where the screen would normally be. I then rested my hands on the screen as if I were typing. The drop in signal is similar to what I observe with the internal adapter on the Yoga. And ironically, if you turn the Yoga upside down and rest the screen on your lap with the keyboard in the air, the signal strength is fine!
I can also confirm that I have the stuttering problem if I use my wifi hotspot in a different room and push the connection to the limit (even though the network works fine for other laptops at that distance/signal strength). So in response to saulcf, it could still be an antenna problem, just not one that appears if you are sat on top of your hotspot or router.
In other words, the form factor of the yoga presents interference problems that you just wouldn't experience with a normal laptop. Normally a laptop's antennas are in the screen with plenty of air on either side. People have these issues with tablets and I'm sure that the ideapad tablets have a similar issue. But when the software is working properly, this simply acts like a slower connection unless the connection is really weak.
But, as I mentioned in a previous post - the stuttering effect happens with the Yoga using different wifi adapters, suggesting that the issue is not solely about the internal wifi adapter. Hopefully the fix for the buffering is a relatively simple software fix because it appears that if that was fixed, the laptop would behave no worse than any other tablet subject to the same environmental constraints. Notice I say tablet and not laptop, because the yoga's form factor and antenna location is going to mean that it is more susceptible than the typical laptop.
For future designs, Lenovo could look at doubling up on the antennas and when the device is in laptop mode, enabling antennas in the screen.
I'm not sure I have any other insights - happy to hear other suggestions if people have them. Particularly interested if someone understands how the buffering of wifi is supposed to work or more technical details about the architecture of wifi on the yoga.
Fingers crossed that we see an update soon and that the fixed device behaves acceptably despite the weaker signal strength (something that may be harder to fix).