I wonder if the best solution wouldn't be to steal the micro-toothed top gear from a narrow range cassette -- for example, the 13T from a Shimano HG50 13-26. Then you'd preserve 7 usable gears (13-40) even after moving the Sunrace 11T behind the granny.
(thank goodness 7spd steel parts are cheap)
Do you mean 13T? The CSM680 specs shows cogs of 11-13-15-18-22-28-34-40T.
Either way, it looks like a very promising cassette. Inexpensive, plenty of range, pure steel, and easy to mod. I'm all about having fewer speeds! Less time wasted shifting -- especially since the GearSensor does not...