I've taken a 7,900 ci pack several times. I take the lid off and and try to not pack it much above the frame so it looks smaller. And I cinch everything down tight so the bag looks as small as possible. I've had a couple flight attendants make a comment on how big my bag is, but no one has ever stopped me (when this happened I was carrying my bag in front of me and not on my back so it looked larger with the waist belts sticking out). I had a buddy that had his pack including a lid FULL on a trip and TSA tried to make him check it. It's not up to TSA how big of a bag you can carry on, it's up to the airline. It's TSA's job to make sure there is nothing dangerous and that's about it. He got it through TSA after explaining it wasn't up to them and then gate checked it without issue.
Worst case they make you gate check it. So like someone mentioned above, I keep optics in the lid inside the pack so I can take them out if they were to ever make me gate check my bag.