My experience is that they're generally flexible, even in economy. Most airlines allow a few kilos over without too much fuss - even BA let met check a 32kg bag on a LHR-HKG award flight once (we all know how full those flights are).

For QR, my biggest concern would be hand luggage. They specify 9kg for economy, I believe. Some airports' agents do make you weigh your bag at check in if they see you carry a large carry on unless you have FFP status. J should be fine.