loud... errrm no! certainly isn't with those fans I can tell you.
I'd advise not to use the downvolt cable, 1. you don't need to and 2. you'll only keep the temps up on the card (add ing the fans is to keep the temps down no?) so you may as well stick with the original fan (which to be very honest does a very good job anyway) than add additional fans and downvolt!
Well loud is subjective, I'm talking about the sp120 high performance, which others have complained as being too loud.
Main reason for fan replacement is for looks not temp. Will probably try with and without the 7v adapter though. (Assuming it's fine to use the 7v adapter?)