In Modern Greek, is there any difference between “I have said” and “I have been saying”?

έχω πει, the perfect tense, is only used in perfective contexts (completed actions); so you can’t use it for “I have been saying”. You will use the imperfect, έλεγα, for that. So Greek makes no distinction between “I was saying” and “I have been saying”.

The English “I have been saying” looks like it’s both perfect and imperfect; in fact, the tense is imperfect, and the use of the perfect in that combination has its secondary meaning, of “present relevance”. If anything, if you want to emphasise the present relevance in Greek, you will end up switching from the imperfect to the present tense, with an adverbial phrase to indicate that the action was continuous in the past. So:

Σου το έχω πει: I’ve told you

Σου το έλεγα: I was telling you; I have been telling you

Σου το έλεγα εδώ και μήνες: I was telling you for months

Σου του λέω εδώ και μήνες : I have been telling you for months (lit. I am telling you for months)