I've been fiddling with this and there's a couple of issues.......
1) The Shimano braking track of the rotor is narrower than the BB7 pads. This means that you are not getting the full effect of the BB7 - which are very good. I set the calipers high on the outer diameter limit, and filled away a small portion of the pad lower so it wouldn't wear the rotor arms that support the braking track.
2) If you use "spider" rotors like RT79, the tab on the BB7 pads need to be cut away. The "spider" rotors are nice and stiff which is actually a problem as the BB7 relies on a bit of rotor flex to work. The XT one listed above I guess shouldn't have that issue.
My advice after faffing and not really being happy with the combo, is simply get this:http://www.chainreactioncycles.com/se/e ... -prod40908
The other alternative is to simply use a full Shimano set-up, and skip the BB7 in favor of possibly an inferior caliper.
Favorite quote on Retrobike:
Not worth the petrol to take it to the tip so it's down to the angle grinder to make it small enough to put in the wheely bin.