Don't know about international flights but domestically I really doubt you'd have a problem. If you haven't bought the MLC yet there's the Eagle Creek Adventure Weekender than can be stacked on top of your rollaboard to look like a big laptop bag. I see plenty of business travelers carrying similar sized bags domestically.

Good luck with stuffing a big "personal item" under the seat in front of you. Even if it fits you won't have much room left for your feet. There's also the question of why one needs 5000+ cubic inches of luggage space. If you really need to bring everything but the kitchen sink with you I'd seriously consider checking my rolling bag...