Great Choice Alex... I have put on over 100miles on my Z since the install of my Berks HFC/CBE.... The HFC's are great with no CEL thus far...as long as the install is done properly you should have no issues with a CEL...
In Terms of how much louder it would get...you will definately hear a difference right off the bat...but IMO it gives the exhaust a deeper more exotic sound...You will definately hear the famous "Hiss" on deceleration, but I don't know I love the Hiss...and its barely noticeable when you're in the cabin....The Berks HFC paired with the Stillen CBE sounds super sweet....Here's a vid of Semtex's Berk HFC/Stillen CBE combo if you haven't heard it yet...
In regards to your choice to go with a Nissan Dealer for the install...I agree with you 100%...especially if your Nissan Dealer is Mod Friendly....When I did the install on my Berks/HFC, one of my friends was a certified Nissan Tech...This helped when we ran into the "Devil Bolts" located on the top part of the Triangle Flange of the Stock Cats....Of course having experience with working on cars, we let off when the suckers wouldn't budge to avoid rounding off the head of the bolts.....Once our friend from Nissan arrived, all he did was break out one of his special tools and with the assitance of a breaker bar we got those "Devil Bolts" out in a jiffy...I honestly feel that it pays to have a mechanic that is familiar with the ins and outs of the Z to work on it...