With the DCS3000 you want to let it sit in 7.5-8.5". The skimmer will take 3-4 weeks to break in. Usually if the skimmer is not handled within two weeks you will get decent filtration but the stability of the skimmer usually doesn't happen till the 4th week of running. I would not use the neck extension till you get the skimmer broken in and dialed into your system first.
How long has the skimmer been running for?