We shouldn't answer a question that wasn't asked (especially by a first-time poster), but it is possible that the OP has Pompeii plans that will be harder than they think. And that relates to the actual OP question.
Well, I thought I was answering the OP's question. As Stated...
Most people have suggested to Train from Rome to Salerno and ferry to Positano and same for the return trip.
If the weather is affecting the ferry schedule, would my next option be catching SITA bus from Positano to Sorrento and then Italo train from Sorrento to Rome?
My point is that traveling from Positano to Rome via Sorrento and Naples is not the next best option, that traveling via Salerno would be better, a point of debate I suppose, but at least my opinion. (I guess I did not point out that there is no Italo train from Sorrento..my bad)
Also, not sure where Pompeii enters into the equation, the OP is seeking advice on going directly from Positano to Rome. As for a taxi in Positano to the bus stop, I indicated that "depending on where your hotel is", I have no idea where they are sleeping, but I would probably choose to go downhill to a taxi stop and take the taxi up, than climb from the bottom up, I guess they may be staying next to the bus stop as well.