What do you consider "up to par"? What, for you, is an acceptable level of DPS difference on single-target fights between the specs?
We're already in a situation where all three specs, if played optimally, are close enough to one another in single-target damage that the vast majority of players will perform similarly well regardless of the spec they choose. That doesn't appear to be changing much in 5.2 -- if anything, the DPS tweaks being implemented are geared specifically toward keeping them close.
If you're a highly competitive raider or in a bleeding-edge progression group in which everyone is squeezing DPS from a stone, there will almost inevitably be a "best" spec for a given encounter. "On par" is irrelevant in that scenario; you're either playing the best spec for that fight or you aren't (depending on variables such as fight strategy).
The extent to which one spec is "close enough" to another in terms of DPS potential is entirely subjective, and as likely as anything to be based on what type of end-game PvE a person playing a rogue is involved in.