Yes, it's perfectly normal. Metro: Last Light is a very demanding game that uses a lot of the latest DirectX 11 effects. Metro 2033 was the same way. I recall hearing TotalHalibut say on his review of the game that he couldn't max it out with two GTX Titans, so two 680s not cutting it isn't very surprising to me. Perhaps some future game updates or driver updates will increase performance, but in the mean time, just turn down SSAA to x2 or even off, then everything should run a lot better. You don't need that much AA at 1440p anyway, even if you do want the game truly "maxed out".