I've got a benchtop PSU powering a USB charge only cable feeding the Bionic.
Keep in mind, most USB cables only output ~500mA. The stock Bionic charger outputs 850mA.
Now, if you use any sort of 4gLTE over an extended (2 or more hours), you'll notice the battery drain very quickly, especially with the stock battery. With my benchtop PSU I've measured a draw of ~ 1200-1300mAh while tethering.
This means that even if you are plugged into the 850mAh charger, with high usage the phone is still draining. This is especially true during USB Tethering with a 4gLTE connection, since USB ports only output 500mAh.
There is a measured MAX current draw of 1500mAh @ 5v while charging the extended battery by slowly increasing the current on the benchtop PSU. The phone won't take more than 1500mAh of charging current.
You'll need a 5v power supply and and old micro USB charger. Splice the two together and you've got a high output charger. Use a 1.3A-2A power supply.
I've noticed that even with max performance battery settings, irregardless of Stock or Extended, at around 20% battery the Bionic will cut all Data connections (most likely to conserve power.)
With the new charger you should be able to charge the phone AND use it at the same time. It will also cut down on your charge time.