Assassination(Sin) is top-end (aka within top 5 on ST fights), but you have to understand that the other specs Sin competes with, are MILES above Sin in cleave and AOE.
These specs include, but are not limited to, Frost DK, Fury warrior, Demo Lock, Aff Lock, BM Hunter, MM hunter, Havoc DH and Frost mage.
Out of these contestants Frost DK, Fury warrior, Aff Lock, BM Hunter and DH completely destroys Assassination on cleave fights. It's not even funny.
Also, the only reason we are even on the meter in the first place, is Agonizing Poison, which enables good scaling for the spec.
For comparison, my rogue (918 eq ilvl) gains 25 dps pr. mastery point, 18 pr. vers point, 14 pr. crit point and 10 pr. haste point.
A DH of equivalent ilvl gains 30 dps pr. crit point, 26-28 pr. mastery point, and above 20 dps pr. point for both haste and vers.
As you see, DH will (and does) outscale assassination by insanely much.
So far, few changes have been made to DH, meanwhile they are going to attempt removing Sin scaling.
The other rogue specs scale worse than Sin in almost every way. Especially Outlaw (due to passive crit, energy and attack speed from buffs, among other things).
But surely tomb has a soak mechanic, where we can use feint, don't you think?
Incoming people who say Sin is OP, but haven't considered the fact that this spec is quite similar in ST dmg to the aforementioned classes, but COMPLETELY and UTTERLY useless in any kind of cleave scenario; for example mythic+ dungeons.
- Ok, useless is a stretch, but compared to these other classes, you might as well just afk on trash packs. I've defaulted to outlaw in m+ for the time being, and it performs quite decent, assuming you get rolls.
So basically, unless this AP replacement is not happening, or if it's a really good nuke ability that does both good ST and AOE dmg, then assassination is probably in the gutter for Tomb of Sargeras.