I went through the EC interview prep (over the phone), and the DC10 sim prep. EC told me that in fact it would be best to not volunteer that I had gotten interview prep, unless I was specifically asked about it at the interview. Well, they did ask, and obviously it didn't matter that I, or 5 of 6 of us interviewing, had gotten the prep. We were all hired. They were just keeping stats for some reason. Anyway, long answer to your question, but no, getting the prep will not help in getting you the interview, IMO. However, as you noted, as soon as you get the call, get the prep set up ASAP.

Good luck.