I believe the claim was that miraculous events were inserted into the Gospels – not the “canonical texts”.
Gospels
are canonical texts.
In any case, I’m not seeing how two ambiguous references to “mighty works” performed by Paul in his letters – without a single reference to any of the many miracles performed by Jesus in the Gospels in those same letters – in any way demonstrates that the Gospel stories weren’t embellished.
Perhaps I should explain, then. I’ll go slower this time.
This isn’t a proof by way of reference; I’m not saying that Paul is referring to miracles that are present in the Gospels and that a direct link from Pauline text to a miracle by Jesus in the Gospels can be drawn.
Rather, I’m pointing out that the claim is that miracles were only added later on in the game – without putting too many words in the OP’s mouth, the typical claim is that later editors (in an attempt to ‘spruce up’ the credibility of the divinity of Jesus) added these miracles to the text out of thin air (as it were

). If that’s the case, then it’s difficult to see how one might claim that Jesus’ miracles were later additions, but that the miracles of the apostles (which were, of course, only possible through Jesus and in His name!) were present much earlier in the texts! In other words, if Jesus were only performing miracles in later manuscripts, how could it be the case that his apostles were performing miracles in earlier manuscripts? That is to say: if the case is being made that Gospel miracles were later additions, then one would be forced to assert that apostolic miracles were later additions as well.
This brings us to Paul. Luke tells the story of Paul’s (and Peter’s) miracles in the
Acts of the Apostles, his ‘sequel’ (if you will) to his Gospel. Luke was written after the other synoptics, but before the Gospel of John. At the very least, it was not the first Gospel to be put into writing, and therefore, Acts was written even later (given that it tells the history of the early Church). However, in the context of the claim we’re discussing, we’re still talking about the timeframe of the construction of the Gospels, not the ‘later’ timeframe in which the miracle stories are said to have been inserted by later editors.
And, we see in Paul’s letters that he himself references the “mighty works” (this is the phrase that is translated as ‘miracle’) that he performed. Now, he doesn’t go into detail; but then again, he’s not writing a narrative or a piece of historical prose – he’s writing a letter to a specific audience who already knows the truth of his claim. What’s important is what he says and how he says it: his claims are of actions that his audience already knows are true; he’s simply reminding them of the actions. What actions is he referencing? The types of actions that Luke (who
is writing a history!) details in his narrative. Luke records the apostolic miracles that Paul directly references in his letters.
So: if the claim is that miracle stories only entered the Gospels at a (relatively) later date, then we have a counter-example that demolishes that claim: Luke writes about apostolic miracles – which can only happen if a divine Jesus, too, performed miracles – in Acts. Paul references his own miracles in the letters that are the earliest of Christian writings. In other words, the assertion doesn’t hold up. (Not through a direct citation of the Gospels from Paul, but through his words and their substantiation in Luke’s account.)
Hope that helps.