I think that Postquam Graeciam veni philosophiam didici and Cum Graeciam venissem philosophiam didici do mean the same thing.
According to Woodcock #215, the most common subordinating conjunction indicating "time after which" is cum, followed by postquam, posteaquam, post ... quam, postea ... quam, ubi, ut, simulac, simul atque, cum primum, ut primum, and quotiens. All of these conjunctions normally use the indicative (just as you did in your question), except cum, which has a chapter of special rules all to itself.
#235 describes the evolution of these rules, which culminated in always using the subjunctive when "after" is meant, and it is always the imperfect subjunctive or the pluperfect subjunctive which is used. So your cum venissem seems exactly equivalent to postquam veni.
Is it applicable to antequam?
In #225, Woodcock describes conjunctions used for "time before which". He mentions priusquam and antequam, but does not mention cum.
However, quite a bit later in #237 he mentions the cum inversum, a co-ordinating conjunction meaning, basically, et tum. This (I wonder. He doesn't say this.) might possibly be interpreted to mean "before", if you squint. Maybe.
First he gives Hannibal iam subibat muros, cum repente in eum erumpunt Romani (Hannibal was already approaching the walls, when the Romans suddenly sallied out against him).
Next he gives Caedebatur virgis civis Romanus, cum interea nulla vox alia illius miseri audiebatur nisi haec: "Civis Romanus sum" (He was being scourged with rods - a Roman citizen, and meanwhile no other utterance was heard from the wretched man except this: "I am a Roman citizen").
In #238, he has this mouthful: permulti anni iam erant cum (= per quos annos) inter patricios magistratus tribunosque nulla certamina fuerant, cum (= inversum) ex ea familia certamen oritur (It was now many years since there had been any quarrels between patrician magistrates and the tribunes, when a quarrel arose from that household).