"Twilight herself said at the end of the episode that she had "more magical talent that anypony she has seen", also saying that "her magic couldn't stop her and she had to convince her to stop on her own"."



Hmm... maybe TS said it because she didn't want to use "unsavoury" methods to stop SG?

That's far-fetched, I know. But well, it could be. After all, SG, at that time, didn't seem to show much in terms of restrain, did she?





"How is that Twilight is more powerful than her and still couldn't stop her?"



More powerful doesn't mean invincible.





" If that's not overpowered, I don't know what is."



Sure looks like it, indeed.





"And if you mean the battle with Tirek when you say "what TS pulled so far"



Yes, that. And all her other feats.