Brand new weapon that may suck ?!?

We had another wednesday coming our way. And a new boss in the Coliseum to look at and faceroll. Like we have did with the others the last couple of weeks. This night it was the Twin Val'kyr's, or also known as Fjola Lightbane and Eydis Darkbane. I must admit I had not read anything about this boss, or seen any video. It was a whole new experience for me this boss. Our RL went through some tactics short, which I must admit I didnt understand alot of. But in they came, and we got em down 20% or so when we wiped. But enough to ppl got the hang of the fight, as we got em down in 2nd try, very easily.

Ohh, well that was not the highlight of the night. Or maybe it was. On the 2nd boss in there, Lord Jaraxxus, a weapon dropped. This is the piece, Lionhead Slasher.
Nice I thought, a good sub to my current offhand, Peacekeeper blade. Or atleast I thought. I know that you should go after a slow weapon, but it was a good deal higher in dps, so my initial thought was it would become an upgrade. But it didnt give me a huge upgrade in dps, it gave some or atleast so it seemed. But curious of others opinion, I asked the question on EJ forum's.

Here is the response.

You could probably switch to an unholy DW build for the time being since you have a nice offhand like that or even one of the other unholy DW builds that doesn't require a slow MH. It's a great weapon to have but you just have to decide on whether you'd like to switch up your spec to really use the weapon effectively.

and

Not only slow ulduar weapons would be an upgrade. That weapon is about as good as a slow blue reptuation weapon (like [Reaper of Dark Souls]) . If i remember correctly from my deduction a few posts back going from 2.6 speed to 1.5 speed is only worth it if the faster weapon has about 60 DPS more. That means that Lionhead Slasher is upgraded by anything from naxx and beyond with a speed of 2.6 and slower.

Something to think about. What I have done for now is get my dualspecc as a dual unholy build, to try it out. Atleast untill I get my hands on a new weapon again, a slow one.
Here is the build im trying out, Build. Lets see how it all turns out. I have only done a few Target dummy test's, the first was in my Frost DW build. Made 2669 DPS, afterwards I took the Unholy build for a spin. There I ended on 2710 DPS. It was a very short time, so the test aint very accurate, gonna have to test it in live raid action to see how it goes.

Any have any pointers here ?