If you plan to use your 8-15mm at the 8mm end, you will need to remove the shade. The shade will vignette when you are shooting full circular fisheye.
Aquatica do offer a version of their port (by order) that has the shade machined off.
My understanding is that the Zen DP-100-ACR does not need an extension. I have used it with another housing brand and it did not.
Unless someone comes along who has personal experience of using this lens/port combination, I would follow the advice of your local dealer.