I was chatting to a test pilot recently who stated that the performance calculations really do depend on ideal conditions and it does take very accurate flying, no detrimantal gusts and an 'average' (gross margins etc) aircraft to achieve the published figures. This all surprised me somewhat and after our conversation I definately came away with the idea that in the event of a V1 failure I would use whatever thrust was available!
The published figures do work but with absolute minimum room for error.