Lufthansa has a 8kg limitation, so I'd be quite happy with 10 ... and that's nothing compared to BA's generous 23kg cabin allowance.
The question is not "how strict are they ?"
the question is "will they check my bag, on the day I fly, even if I'm the only one singled out and checked ?"
Always pack assuming they'll check, or be ready to face the (financial) consequences if the airline decides you need to check your bag.
I cant' answer for Norwegian, but on LH flights for example, they have agents casually roaming about the check in, and boarding areas. And if they suspect, a bag is overweight, or oversize, they'll take you out of the line, and lead you to a sizer and scale. We've seen some last august, including at MCO where they tagged the cabin bags with "approved cabin bag" tags.
now this is LH, but consider this could be the same for any airline, and it will become more and more frequent as time goes by.
so as I said above, you may get away with it, and you may have in the past. It's not the question here. The real question is : If I play the game, am I ready to accept the rules and am I ready to face the penalties in the unlucky event I'm caught bending them.
Now, at the end of the day, it's your decision.
As far as I'm concerned, I'm comfortable with a bag that's one inch larger than the posted sizes, because I know it will fit, and since they're soft bags I do not overpack them and they're easy to squish to fit in the sizer (which will most of the time be a bit larger than the authorised size.
But I do not push it farther than that. And we do travel with carry ons only on the way to FL, so size and weight are a case of playing Tetris puzzles when packing, especially with the 8kg at LH. Well, I always pack like they will check my bags. Peace of mind.
