Farming missions by replaying them over and over is not an exploit in any way. What people do not understand is that "Time" is the only way to rank up fast. After like an hour, you may be bored and give up and start looking for new ways to rank up. In reality, the only way to rank up is playing non stop. When GTA online was first released, I was level 120 in about 6-7 days. We kept replaying missions (Not rooftop or anything exploit) over and over for 15 hours a day. We were so bored but it was worth it in the end. While everyone was level 10-20, me and my friends were 100+, We were called modders/hackers/ext. We just took the time and effort to rank up. We played a 6 player mission called 4 trailers for 6 days strait in the same crew. So we got extra rp each time. But there is no other way to rank up fast without exploits except by taking the time to actually do it. Me and my friends did it in a week during release, can you?

That was i usually was doing, but they removed the ability to replay missions from the characters. Is that mission one of the ones you can start from the menu?