They are similar in sims, but in practice? I had no problems ranking on stuff back in t14 as assassination, but this tier as combat, if I were competing for ranks on the assassination list, there are quite a few fights where I would be below 90% of the rank limit or more (now this still gets me really high combat ranks). It could be that I am just too low on gear; I had the 4pc for a couple weeks but my ilvl was only 515 until I got crafted boots and the heroic jin rokh helm tuesday putting me still only at 519 after which I havent killed anything.
To be fair, I generally handily beat our other rogues (2 of which are assassination), but we all know 'omg but i beat everyone in my raid as X spec' isn't really useful information. I'm thinking about trying assassination, but we have had 2 daggers drop (1 TF) the entire tier and I don't really want to put in on a dagger when our assassination rogues really need them just so I can compare specs and maybe switch (have a 522 combat weapon, think everyone has at least 1 that uses 2.6 agi weapons now so maybe a 2nd soon).
This a hypothetical situation or are you suggesting combat dps is under tanks?