If I'm not mistaken, IGN has complained about the frame rate in pretty much every single 360 RPG I've ever enjoyed (Lost Odyssey, Blue Dragon, Mass Effect) and I've never found the frame rate in any of those games to be any worse than a minor annoyance (and usually I don't even notice it). I'm not too worried about Infinite Undiscovery.
On the other hand, I don't know how when I'll get around to getting IU. Money's tight & I still have mostly untouched copies of Rogue Galaxy, Odin Sphere, Devil Summoner, and Valkyrie Profile 2 in the Action/RPG category.
Valkyrie Profile 2 is not an action rpg. its more of an evolution of turn based rpg's. also i often see people say shadow of the colossus had frame rate issues but i never saw it personally,at least not on the scale of dynamis in ffxi.