It's probably intentional that classes should complement each other's dps so that it becomes more effective to have a mixed group, rather than encouraging people to just stack, and play, only one class.
RKs for example have their armour debuff. If you grouped the RK with a champ, then the armour debuff amplifies the champ by a large %, and rend amplifies the RK. The most likely outcome is that 1 RK + 1 champ is a fair bit higher dps than 2 RKs, even though solo RKs have the dps edge.
If you get 4 different dps classes the result might be even better because each member buffs and gets buffed by 3 other members. While in a 4 RK group, each RK just buffs himself (if you don't count higher mit debuff uptime).
I've come to the decision that the most rational response would be to look at those videos and put a finger on why the good ones are doing well. The recent 4 man SS t2c vid that you mentioned looks fairly representative, so this will be what I'll be looking at below. In order of appearance (RK PoV starts at 4m50s):
- The first thing he, the elf RK, does is to start with a Writ so his skills can proc t1 Searing Words while the dwarf RK stays at t0, and they end up with all 3 early-tier Searing Words within seconds which is perfect.
- His starting rotation for bosses looks to be Essay + Scathing Mockery, Writ, Distracting Flame, Smouldering Wrath.
- Of all the dots which RK's have, Scathing is the weakest by some distance. Fiery Ridicule's dot is twice as strong and even Essence of Flame without mit debuff would've been more effective, so either of these after Essay #1 would've been an improvement.
- When Smouldering Wrath ends he goes for Fiery Ridicule even though he could've gone for Essence of Flame. He does do Essay + Essence 3s later, but it's not the ideal approach.
- A ghost spawns and the dwarf RK puts a t1 Writ on it which results in mit debuff, then both RK's burn it down with Essay/Smouldering so it's dead in seconds. That's how it's done.
- The dwarf RK used his first Writ 30s after starting to attack. Waiting so long was wasteful because t0 Searing procced within seconds. Overall he's much stingier with Writs than the elf RK so most of the time they're on different Writ tiers, which might have been coordinated.
- When the lights spawn in the elf RK was already in position. He does Essay + Essence on one light, swaps target and does instant Writ and Smouldering Wrath on the other which procs mit debuff, both dead within seconds.
- Back to boss, where he's doing mostly Fiery Ridicule chainspam. I'm guessing he was distracted by Teamspeak.
- A new ghost: this time Essay + Essence, Smouldering Wrath and gone. Probably the fastest way to kill weak adds.
- Side bosses spawn. The elf tiers up Writ t2 right at the start even though there was no Searing Words and he had the mit debuff, and that gets repeated again later on vs. the main boss, so it looks like the elf RK isn't considering Searing Words.
- It seems like he sometimes waits with Essence of Flame until after Essay and instead uses Fiery Ridicule. This doesn't make sense.
- Seeing lots of Writ-Ridicule spam at this point whenever other skills are on cooldown, always starting with Writ, which is an optimal use of time.
- Boss is almost dead, so starting from 0 attunement Essay + Essence, Ridicule, Writ, Smouldering. The Essence impacted without mit debuff so would've been better if it came short before Smouldering, but on the whole this is minor.
Overall the team worked well, as the whole 4manning thing already kind of shows. They didn't seem too concerned with Searing Words which isn't too good considering how often in the fight you take breaks from attacking the main boss, but the fact that the 2 RK's were at different tiers of Writ most of the time probably made a noticeable difference.
Rotation wise there were some inefficiencies (i.e. putting Fiery Ridicule before Essence midfight, putting Scathing Mockery so early) but things like those are probably just a handful %'s of dps. More optimal if you do it one way rather than the other, but not decisive.
What probably made the difference was that they were efficient with adds: I'm citing the part after 5:25 as the best example of this, when Gildra got rid of lights with Essay + Essence on one light, then instant Writ (mit debuff) + Smouldering Wrath on the other. It looked like they knew when and where the adds would be coming so they were already waiting in range and prepared to attack. Often they started with Writ/DF, then followed that with Essay/Smouldering so most adds were killed in about 3 skills. What I take from this is that knowing the instance well and dealing with adds efficiently is by all chances the single most important factor in dps for anyone who's got their rotation down.
Edit - quote from a message I sent recently, concerning adds in SS t2c:
These long rotations - conventional and especially advanced - are mostly intended for long-lived enemies like bosses. For weak adds my preferred tactic is to start with DF, Writ for mit debuffs, then follow with Essay + Essence, (2nd Writ if still no mit debuff) and Smouldering Wrath. If another RK helps you that's enough to kill the ghosts, meaning you don't need to do Writs & Ridicules afterwards which would pull down the average.
Dosage is another major factor with adds. If an add already has lots of dots and is low on health, you probably don't need to spend any more time on it. It's a matter of experience to judge when you can move on to the next add.
Similar to the above point, one of the dps traps is overkill: Essay + Smouldering even though just one would've been enough. Of course if the add must die asap you'd do both, but otherwise it's better to set up something like Essay + Essence/Ridicule, Writ, (DF) on one add and use the Smouldering Wrath for something else.
One example of the above point is with lights. It doesn't get any more efficient than Essay + Essence on one light, then swap and Writ + Smouldering on the other light.