ἐπαχύνθη γὰρ ἡ καρδία τοῦ λαοῦ τούτου, καὶ τοῖς ὠσὶν βαρέως ἤκουσαν, καὶ τοὺς ὀφθαλμοὺς αὐτῶν ἐκάμμυσαν· μή ποτε ἴδωσιν τοῖς ὀφθαλμοῖς καὶ τοῖς ὠσὶν ἀκούσωσιν καὶ τῇ καρδίᾳ συνῶσιν καὶ ἐπιστρέψωσιν, καὶ ἰάσομαι αὐτούς.
If I'm not mistaken, μή ποτε here isn't a negative word--it means "perhaps." "Perhaps they might see with their eyes and hear with their ears and understand with their heart and turn around, and I'll heal them."
I can't see how μή ποτε could be interpreted as a negative here--in fact, he's suggesting that they might
do all of the things he mentions: see, hear, understand and turn around. So there's no reason to repeat the negative with each verb (as there would be in Matthew 7:22). In fact, adding a negative particle to each verb would not just be strange, it would convey exactly the opposite of what is being said. http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0057%3Aentry%3Dmh%2Fpote
In short, this passage (Matthew 13:15) is not a valid parallel to Matthew 7:22, and, to repeat, if piro's translation were correct, we would expect the parallel verbs ἐξεβάλομεν and ἐποιήσαμεν to be linked by οὐδὲ, instead of καὶ.
Added to that, as previously mentioned, οὐ at the beginning of a sentence is a typical way to introduce a yes/no question expecting a positive answer. Instead, to their surprise, the false prophets get a negative answer.
And this follows: ἀποχωρεῖτε ἀπ᾽ ἐμοῦ οἱ ἐργαζόμενοι τὴν ἀνομίαν. οἱ ἐργαζόμενοι τὴν ἀνομίαν suggests that he's responding to people who have done something wrong, not to people who haven't done anything.
I should disclose that my background isn't Christian, so perhaps I shouldn't be interjecting my views here.