First time I've heard him described as such and not the other way around. Never thought of Leamy as an 8.
I don't think POM is a seven though, just like SOB isn't a seven. Any of the turnovers I've seen POM get has come from being the second or third player into a ruck, and winning it after pressure from the first entrants. A seven's job is to be the first at the ruck first and foremost, he should slow down the ball, and create the possibility of a turnover whether it's a turnover he secures or whether he creates enough commotion that the later ruck entrants can get the turnover. Most of all the seven should be ensuring that the attackers don't get away with putting in minimal ruckers. The seven's job is totally defense orientated in that sense, he should be first at every second ruck. And that's also why sevens can turn over a lot of ball, they're first there so they're going to pick any 50/50 rucks. POM is rarely the first at rucks, he can clear them, but he's usually second or third and that's where he wins ball. And he wins ball that way to create his own attacks from it, either from the base of the ruck in a pick and go, or being available to the 9 for a pop pass.
Similarly, the seven needs to be able to close up the openside backline gaps. They're usually the fastest, but not really the biggest of the backrows. When the eight is the attacker, a big runner, they can run at the 9/10 channel from a scrum. And it's the opposing seven who needs to be able to close that space off. The defending 9 isn't big enough to stop a full flowing 8, and should be watching the attacking scrum half anyway (and is usually impeded by him when the attacking eight runs, anyway.) Similarly, the defending seven needs to be able to put pressure on any backline movement from a passed scrum, because attacking backline movement has the most space to work in off a scrum/lineout (and lineouts are another case where a seven will be the first man left out in a shortened one.) POM's strength is in his attack, he carries well and makes yards, but he isn't big enough for an 8, and his defending is good enough to deal with an opposing eight going into the blind side, but not good enough for the kind of backline move you might get off a scrum, so six is the best spot for him.
To be honest, I'd have to see him hitting a lot more rucks as the first person, and making far more of the critical tackles in defense before I'd consider him above O'Brien, because at the moment O'Brien is far better in attack than POM, and has proven himself extremely reliable in defense, even if he isn't the typical seven. So if we're going to play a non-typical seven, then we might as well go with the proven better player. At least until POM shows he has game in more places than O'Brien.