If a raid leader/guild leader is bad enough to use that one line blurb from this thread as a standard by which they judge their Death Knights, then they're not competitively raiding anyways (or they're not intelligent enough to last attempting to do so). That's not the purpose of that line, and if anyone (DK or otherwise!) believes it to be so, you are quite mistaken. As far as judging one's performance, comparing parses is always the best way, and what most top raid leaders/guild leaders know to do (or, at least, should know to do).
With no real way to realistically compare the specs in current PvE content, the only answer is to use a sim unfortunately. In doing so, I would probably add a modified figure for RP gain to reflect magic damage that can be soaked by AMS (Unholy only). However, using anecdotal evidence and parses from WoL now are pretty useless as DPS varies wildly between the two specs depending on the fight.
I do mimic AMS soaking for Unholy.
At any rate, Theodos is right. The fact is, each fight changes. Some fights have AoE; other's don't. Some fights have 100% uptime on the boss; others don't. Some have +damage modifiers, some have target switching, some have silences... other's don't. No two fights are identical, and thus when comparing two specs you're going to have to make assumptions which simply won't apply to every fight. It's unavoidable, unfortunately. Short of going through each individual boss and saying how the specs perform relative to one another (which, actually, is quite doable, if people wish), when using one set of numbers, a "Patchwerk-esqe" set of assumptions is how it has to be.
Besides, it's much easier for someone playing Unholy to reach their theoretical max dps than it is for someone playing Frost to reach theirs, and that's largely because of RE.
As to parses, comparing some of the top ones for BWD shows...
Omnitron: Unholy is ahead by ~18%.
Magmaw: Frost is ahead by ~21% (ignoring the 42k dps Frost outlier!).
Chimaeron: Unholy is ahead by ~5%.
Atramedes: Unholy is ahead by ~14%.
Maloriak: Frost is ahead by ~4%.
Nefarian: Unholy is ahead by ~18%.
On two of the six fights, Frost is ahead, and on four of them, Unholy is ahead. The smallest gap between either of the two specs on any one fight is about 5%, while the largest is near 20%, and said gaps (both the minimum and the maximum) go each way for each spec.
The only conclusion I would draw from those parses themselves is that it all depends on the specific fight (as it always has and likely always will). On some fights, Frost will steamroll Unholy by a ridiculous margin... while, on others, Unholy will blow past Frost by a similar number. On some fights, Frost will barely eke out a win... while, on others, Unholy will just barely manage to do the same. All depends on the fight.
Sure, one could argue that the fact that Unholy wins more fights ultimately makes it the better spec, but that's silly reasoning. If you're simply looking to do the best you can on every single fight, you're going to be dual-speccing and constantly switching, as always. And if you're not quite that min-maxing, then you're most likely just going to concern yourself with which fights are troubling your guild the most, and spec appropriately, not which spec will make you look the best on the meters on fights you've been farming.