TurnerGuy +1
There's a lot of experienced riders who are using BB7s and there are good reasons which I won't enumerate.
What causes poor performance with good quality cable disk brakes is usually quite simple, flex or compression.
Compression:
The performance of any cable brake is subject to the quality of the cable. You can buy the best cable brakes possible, combine them with poor quality cables, and you end up with crap brakes.
What happens when you pull on the brake is the outer cable compresses (the spongy feel) and good cables compress less than rubbish ones. So you have used up a heap of your lever travel just to compress the cable to the stage it is ready to apply the brake.
With crud cables this means you have to set your pads really close to the disk, so then you get the delights of rubbing brakes etc.
I have had good results with Jagwire and Avid cables. (I've nothing against Shimano cables, it's just I haven't used them in years.)
Flex:
This can be found in some brake levers. Once again you are using up cable pull to flex the lever to the point that the brake is ready to work, and again this means all your adjustments leave the pads too close to the disk. It is worth buying decent levers. Avid, Shimano, etc. If you can see your lever flex it's too floppy.
If you are using road BB7s then of course you have the problem of using road levers. Nasty noodly things IMO, but you can make up for that by getting rid of all the compression in the outer cable.
This is done by replacing outer cable with rigid metal tubing. Avid make an expensive kit, or you can make your own like I did using aluminium tubing from B&Q - it's cheap, and some skinny nylon or teflon tubing to act as a liner. You'll obviously need some ordinary outer for the flexible bits as well.
Here's mine:
You can see how the aluminium tubing has been curved to follow the line of the frame.
Thus if your BB7s are crap, blame your cables and levers first.
