I've just been through the same process deciding what to put on my Roadrat. LX V-brakes which came on it were ok but I did get through a frightening amount of brake block on my commutes in when the weather was wet. So far I've gone for an intermediate solution of a BB7 on the front and left the LX V-brake on the back. Will probably stick a BB7 on the back in the fullness of time but will have to have a good look at how it will fit around my mudguards.
Wish we'd had this thread when I was doing mine. Scratched my head a bit with the Avid mount before turning it upside down - can't understand how you managed to fit the caliper and have it touch the disk at all with the mount right way up? Don't understand why you needed to shim out the caliper? Didn't need to shim the BB7 - just used the regular tri-align and the slack in the ISO mount adapter