I'd like to see an 80mm fan mounted to this:
I don't think so. It might be able to mount to the side panel, though:
But realize that mounting the fan to the side panel and mounting it to the inside end of the duct would be two very different situations. The flared portion of the duct, in fact, isn't very securely attached to the flanged mount (heck, just look at the pic.), so even if you managed to tape, glue or somehow get an 80mm fan attached to the flared part, there's still a great likelihood that the entire assembly (fan+adjustable interior duct piece) would simply fall off, landing on something--like your video card.
Believe what you want; some few words they typed, or my words and my pictures.
I can't comment on if it would be silent because I don't know what CPU, mainboard, HDD and video card you're using. A CNPS7000B-AlCu and ATI Silencer both at full voltage will still be on the noisy side, and if you're using a Prescott plus an X800XT PE, then chances are, you'll be relying on the full voltage available to those fans. A ceramic-bearing WD Cabiar will definitely be a loud, screeching noise source, damped or muffled or not. Give us more info, we'll be more precise.
Just wanted to add that the reason why having just a fan in the side panel is almost worthless compared to being used with the duct is that, much like any case that just has a side panel mounted intake fan, most of the air will either flow into the PSU (likely a good thing; reduces the chance of ramping) or just flow right out of the back of the case, in both cases, doing little to cool the CPU down, as it hasn't even a chance to flow across the cooler. With the duct, it really helps to force the cool incoming air to hit that sink before moving on. Intel sometimes does weird things, but they did specify the Chassis Air Guide as a duct, rather than just an intake fan, for a reason. Without the duct, that side panel opening is nothing but another place for noise to leak out.