First, I will say that I am nowhere near an expert on frost DK's, I play mine mostly for fun and all, but I looked mostly at the elegon fight, as you said.
One thing that is better to do rather than just comparing you to your whole raid is comparing you to the other 2 frost dk's in your raid, which are both dual wield DK's, and honestly, yes you are slightly under them dps wise, but you are still really close to them overall.
One did I did notice going through the logs is that you don't seem to be using your trinkets and cooldowns enough. For example: You have 2 on use trinkets. You used your Jade figurine twice during that elegon kill, when the trinket has a 1 minute cooldown. On an 8½ minutes fight, you could've used it up to 9 times. Same thing goes for the lessons of the darkmaster, you didn't even use it once during that kill. IN the same range of ideas, you used your pillar of frost 3 times, and I'm pretty sure that this 20% increase in strength could be pretty useful.
One thing that is pretty nice however is your diseases uptime, especially on blood plague. It looks much better than the 2 other Dk's.
As I said, I am no expert on frost dks, but I am pretty sure that if you keep both your trinkets and PoF on cooldown, you're gonna see a pretty decent increase in your dps.
If you keep forgetting about your trinkets and PoF, you can just macro them into Frost strike (that's what I did on my DK). It is not optimal, of course, because it would be a bit better if you could choose when to use them axectly, but it is still better to mindlessy use them like this than to not use them at all.
Of course, if I said any nonsense, anyone feel free to correct me
I hope that this was at least a bit helpful for you, and if you have any questions or comment I can answer, I will gladly do so