So some time ago, after my gushing (gushing!) on the well-done DRK 30-50 line, someone asked me if there was any correlation between the Inner Beast and "Fray" (your Shadow / Dark Side). Now that I've finally gotten around to finishing WAR 30-50, I can answer that.
Yes there is!
In the WAR 30-50 line, Curious Gorge is training you, or at least imparting techniques that he knows while you help him search for his brother, Broken Mountain. However once you fend off some monsters at Wineport during the 50 quest, Curious Gorge fights a berserked Broken Mountain in the city and crushes him by giving in to his own Inner Beast. Before the battle he rants about how great he is, about how he's the true Warrior, about how he's going to bring his village back to glory, etc. After you manage to overcome him, however, he admits to you that he was deluding himself - he knew all along that Broken Mountain had succumbed, and that the way of his village was wrong because it preached using rage to solve everything.
That self-delusion is the key.
Throughout the DRK 30-50 line, "Fray" constantly chides you for being heroic and self-sacrificing despite the fact you get nothing in return, as if you're deluding yourself into thinking that's what you want and/or that people are good. It comes to a head in the 50 quest, where your Shadow manifests independent of Fray's body and challenges you to overcome it.
It's very similar to Curious Gorge succumbing to his Inner Beast - the only difference is that your Shadow is an independent manifestation, while the Inner Beast lacks this ability. However if Curious Gorge was a DRK instead I can only surmise that his own Shadow would behave quite similarly to how he was after succumbing... with less implacable rage, but even so.
That said, there is a fundamental difference between what drives a DRK and a WAR: WAR is driven by the primal lust for blood and violence, while DRK fuels itself with hate and rage toward injustice. Their powers are also significantly different, but that's plainly visible in their gameplay mechanics.
So yes, there is likely a strong correlation between the Shadow and Inner Beast, even if that wasn't intended from the first.