I went for an extended ride with my inexpensive db killers in their "loudest" configuration. I can't feel any loss of power compared to the open Brawlers.
With respect to noise, there isn't a lot of difference at idle. Above 7k they are definitely quieter. At cruise between 70-80 the wind noise about balances out the exhaust. Above 80 the wind noise is louder. The intake noise if hard on the gas is louder from about 75 on up unless I am at about 9k or higher in a gear.
I think I might have about the right compromise. It isn't hard to adjust them, just pull the baffles and move a little restrictor plate. At the moment I am wide open.
If you look down the baffle, you can see little holes on the side. I can put the restrictor plate in any one of these. Moving toward the end of the pipe restricts it more when in place. Definitely worth the $30 I spent.
i will note one thing though. The just threaded the side of the baffle and it is easy to over torque them. I am going to play it safe and buy some locknuts and put on the inside once I am pretty certain I have the noise attenuation I need.