Sorry to hear you're unable to get the most out of your upstream whilst streaming content to Twitch.
I've checked and don't see anything in particular that would explain the issue. Are you using a wired or wireless connection? Are you able to test on a wired connection with the hub in modem mode to test for any improvement please?
I alway's run wired, i use the hub in modem mode as i have an Asus RT N56u, i have tried the hub in router/modem mode and no change, i also have 2 friends that are on virgin that have the same problem.
I have problems streaming on Twitch.tv, i am ok for about 2hrs then i start to get bad lag. i have since moved over to www.gaming.youtube.com can tell you i have not had a single problem streaming live since i moved
my OBS quality settings are set as
1280x720 720p at 60fps with a bitrate of 3800 and i have CBR ticked. i streamed for 5hrs last night from 7pm till 12pm i had no Dropped Frames. it was 100%
my internet is 100/6 my net does slow down a lot but it doesn't afect my upload
if i use twitch i can't stream at these settings for more then 1hr. i haven't got a clue why, but if i were you i would give www.gaming.youtube.com a go