There are a few references to Cordelia’s time in Sunnydale in Rm W/A Vu. When confonting the ghost, Cordelia says, “I’m not a sniveling whiny little Cry-Buffy. I’m the nastiest girl in Sunnydale history. I take crap from no one.”
When visiting Cordy’s apartment, Doyle notices her burnt up High School graduation diploma to which Cordelia replies, “rough ceremony” - referring to Graduation Day (Part 2).
Cordelia also mentions the mansion that Angel lived in in Sunnydale in the following dialogue:

Cordy: “Like I’m still getting punished.”
Angel: “Punished. For what?”
Cordy: “I don’t know. For what I was? For everything I said in High School just because I could get away with it? - And then it all ended, and I had to pay. - Oh, but this apartment – I could be me again. Punishment over – welcome back to your life! Like, like I couldn’t be that awful if I get to have a place like that? - It’s just like you!”
Angel nods: “Working for redemption.”
Cordy frowns confused: “I – I meant because you used to have that mansion.”

Cordelia also speaks to Aura, one of her Cordette’s from Sunnydale, seen in Welcome to the Hellmouth.