The Tamrac Adventure 8 is a bag not a backpack so I'm not sure which one you have. I've just ordered a LowePro Vertex 300 AW. What would be really handy to know is how thick (front to back) a backpack can you squeeze into the overhead bins - I might have to take a tape measure with me next time I fly.
It is a Tamrac CyberPack 8 (I knew it had an 8 in the name).
I have found that a backpack carry-on is less likely to be scrutinized than a wheeled carry-on. I recently switched from a Tamrac 692 (roller) to a Tamrac Adventure 8 (I believe). I can carry everything to shoot one lens w/ 2 strobes.
When catching Liat in San Juan, they weighed most carry-ons but did not look twice at my backpack....which was HEAVY!