The DD 10.5" barrel gas port is a whopping .086" and is certainly over gassed. This is to facilitate the function of the lower cost, underpowered "cheap stuff" ammo. The NSWC Crane spec for the mk18 10.3 is .070", that will function reliably both suppressed and unsuppressed and does not require an adjustable gas block.
Since you are adding an adjustable gas block you can tune it and essentially "decrease" the port size of your DD barrel. With that being said, I would think you still require the extractor upgrade. The actual MK18's called for this modification and they had the proper gas port size. I think even if running the optimal amount of gas, it still alters the characteristics of the bolt function.
Here is the actual power point that goes over the MK18.
http://www.dtic.mil/ndia/2003smallarms/john.ppt
Just out of curiosity, any particular reason you are going with the DD barrel and adj. gas block as opposed to something with a smaller gas port?