I think it also depends on how easy your I-485 case was and the evidence that you submitted at the time of your I-485.
Yes, USCIS keeps everything that you send to them, so if you had a very good I-485 case then it would assist a possible weak I-751 case. When we filled my wife's I-751 over a year ago we included pictures from our ceremonial wedding in her home country (we were officially married by a court clerk here in the US, but her parents wanted to do a traditional ceremony there that happened months later), pictures from our honeymoon, a letter from our property manager saying that we have lived there since 2006 and the amount we were paying in rent, a HIPPA letter from my wife's insurance company showing both of us on her insurance, one or two bank statements showing a joint account (there wasn't a lot of money in there since it was an account we didn't use a lot), tax transcripts for 2009 and 2010, copies of my passport info page and the pages where I had my entry and exit stamps when I visited my wife's home country. Personally, I though it wasn't enough for a I-751. However, we had overwhelming amount of evidence for the I-485 so that evidence was also considered when the I-751 was reviewed. Might be one reason that it takes so long because the file needs to be obtained from archives.
Now when my wife filed her N-400 for Citizenship back in December we only sent what was required in the initial evidence as required on the instructions. She also took the documents required for the interview based on the checklist that was sent with her interview letter. However, the officer had her entire immigration file at the citizenship interview, so the officer never questioned or requested to see my US Citizenship documents since copies where already in her file (sent with the I-130 and I-485), and were marked as seen the originals by the officer that did her I-485 interview. (My wife was applying based on the resident for 3-years and married to a USC rule).
