Hi yes I've used Hero 3 silver, Hero 3 black and Hero4 black together with standard backpacks (not the sort with integral screen) also with external USB power packs. Battery life is generally not far out from the GoPro provided information - although that is usually best case, new batteries etc.. Turning off the screen and wifi makes quite a bit of difference.
The backpacks generally double the recording time as they are about the same capacity as the internal battery. If your only getting 20 extra minutes then there is something wrong. I prefer the external USB power packs as they offer more capacity and can be positioned anywhere within the payload container to provide balance.
Most USB power pack work - I've used no name Chinese ones and Anker (which seem a good brand) - I tend to use the metal case 2800mAh or 3000mAh ones - one per camera - more than enough for 4hrs record (together with a charged GoPro). The idea being the metal case spreads the heat generated by the DC-DC converter to the battery and keeps it warm. Keep the power pack well insulated.
Don't forget to get a big SD as well.