To me 'versed' usually has an adverb, typically 'well' but sometimes 'little'; is it really used alone? And to me it means not so much knowledgeable in the sense of factual content, but more like 'experienced,' knowledgeable about how things are usually done: well versed in the ins and outs of regulatory bureaucracy, arms control negotiations, modern politics ...
I'm also not sure about 'proficient,' which may be more typically skill than knowledge: proficient at a language, at playing a musical instrument, at a sport ...
'Have a solid/sound knowledge of (a subject)' is probably one option, but in many contexts we might easily just use some version like 'really know,' 'know really well,' etc.
'To be well up in sth.' sounds BE and possibly dated to me; I only know it from Gilbert and Sullivan.
Modern AE might be 'to be really up on sth.' That would include, for example, memorization of facts and dates: He's really up on baseball trivia; she's really up on medieval history. It would probably be [coll.], used more in conversation than writing.
Another colloquial option might be 'to have something down' or 'to have something down cold.' Those are both similar to 'down pat,' but not quite the same, because 'pat' implies more rattling something off without really understanding it, whereas just having it down (cold) means knowing it without a shadow of a doubt. Those could include, for example, studying or practicing something: I've studied for this test for a week and I think I really have the subjunctive down now; All the pianists need to have the required etudes down cold.
If any of those contexts don't fit in German, perhaps some of you could give some examples with 'firm sein.'