I'm sure plenty of people have done it legit through PvP, but I'm sure there are only a few dedicated players that have leveled that high solely from PvE. When you get to the point where you play an entire NG+7 Scaled playthrough and still don't have enough Souls to level, it gets stupid hard to hold on to your bloodstain and grind out levels. I can't imagine having to play multiple playthroughs just to level up once. I leveled a character to level 280 legit in PvE, I started tiring of it after I would beat 2-3 bosses in a row on NG+7 difficulty, and putting avarice gear on after I beat a boss, and still not be able to level.
The best, non-cheating, but definitely exploiting way to level is to piggyback level off a friend. You host and summon a friend with red summon sign, kill your friend 10 or so times with avarice gear, then level, then you let him host and do the same to you. If you do it properly, it will never take more time to level because you guys are scaling together. It still takes forever, but it's definitely the best way to do it without having to cheat.
There is no real reason to level past 300 though, you can get every stat to its best scaling level and even a couple like Vitality and Endurance to 99. Once the gains for scaling start to tank, there is little incentive to level, and you ruin the online essentially because you are so out of level range of most players.