The “full-efficiency” number is referring to how efficiently those enemies can still train Force Tempering, not how strong you should be compared with them once you reach that number. I can see how the wording makes that unclear.
So if an area has a full-efficiency level of 190, you gain Force Tempering from those fights at the full rate up to around 190. You can keep gaining Force Tempering well beyond that, but those same enemies become progressively worse training as you outgrow what they can teach your body. At 550 Force Tempering against a 190 range, you are only getting a small fraction of the original training gain from them.
The Force Tempering you already earned is not being reduced, though. Your 550 is still contributing substantially more combat power than 190 would. The declining efficiency only applies to how quickly those enemies can give you more Force Tempering. It is not meant to indicate that reaching 190 should put you at roughly the same Power as the enemies in that area.
The later regions are also expecting the rest of the Lower Realm progression to contribute increasingly heavily alongside Force Tempering. That is why reforging into the higher-tier Impressions made such a noticeable difference for you. Force Tempering remains useful, but it is no longer intended to carry the entire combat calculation by itself.
I may change the wording from “full-efficiency level” to something closer to “full-rate training range,” because your interpretation is completely reasonable from the way it is currently labelled.