It's definitely what I'd consider "overkill" but there's no such thing as "too much fresh air" in a ventilated grow space.
As far as noise; fan speed controls significantly reduce fan noise. As mentioned above; a decent duct muffler can, too. Take the proper steps to isolate the fan vibration from the ducting and the cabinet. Use bungee cord to suspend the fan within the box. Use a piece of insulated (plastic/fiberglass) ducting to isolate the fan vibration from the hard duct (you should be using hard duct). If you want to get really crazy about it, you can glue 1" thick acoustic foam inside all of the "hard" ducting.
It's a snap (and some tape) to make your own duct muffler out of a 24" section of 8" hard duct (do not use pre-assembled duct; use the flat stuff), 1.5~2" thick acoustic foam, and two 6"-to-8" flanges. 2" thick foam in a "pyramid" pattern is ideal. Adding a layer of DynaMat (or similar "loading material") will also help. Apply the loading material to the metal duct as directed by the manufacturer then use spray adhesive to apply the foam. Assemble the duct and tape every seam.
Some pointers to help you avoid expensive mistakes - Ensure that your adhesive of choice is formulated to work with styrene/foam products! Most spray adhesives will dissolve foam (like dumping water into a bag of cotton candy). Install any sound deadening products into the duct before forming it into a cylinder (even without having to deal with the adhesive, it's not easy to slip a sheet of 2" foam into an 8" tube). You may know the circumference of an 8" circle by heart but measure the actual duct before cutting the foam.
