But it's still a violent R rated game so it won't be accessible to the larger gaming audience so I can't see it getting more than 7m.
GTA V sold over 75m, an R rating isn't a limiter.
FW - ~2m
Lifetime - 9m
The thing is, we're talking about two different kinds of violence. The reason GTA does so well is that it offers you freedom to do whatever you want in a semi-realistic sandbox. With that freedom, you can choose to be violent or not be, depending on your personality. It's all comical and laughable violence that you don't get nightmares from.
God of war, however, from the title alone, speaks volumes about the kind of violence you see in it. Kratos does such otherwordly sick stuff that it's hard for other people to stomach it. And you have no choice but to go on killing in creative ways even if you're not that type of person. Also, there is no lighthearted violence here. Kratos just screams while ripping heads off. This series is all serious drama with a subject a lot of people don't care about (mythology).