I travel with an Exo 3500 a lot, had one run-in with an overzealous gate agent and had to remove my belt and lid. I took lid as a personal time and took the belt off and wore it. Looked stupid, and it pissed off the agent, but I won. I almost had to gate check the pack that time.
Bottom line. Your backpack does not comply with the size requirements. Period. End of story. Do all you can to make it look small, especially recommend binding down the chest straps and removing the belt before the gate agents scrutinize it.
You may get lucky every time, but there may be a day when you're hosed. Greatest risk is short jumper/connector flights w/small planes and tight overhead, but gate-checking for those flights usually means you can grab it back as you depart the plane and walk down the roll-up stairs at a small airport like Montrose, CO.
Just hope that the day you get busted isn't the day your pack is full of optics and electronics while the baggage guys have their way with it.
JL