There is, simply, no reason for the sprocket bolts to ever loosen. The reasons that they would would include the nuts actually rotating off of them, which is exceptionally unlikely, and the sprocket moving on the hub, loosening the clamping pressure by wearing the two parts. This should not occur, either, and only would if the bolts were too loose to begin with and/or were a poor fit in the hub.
The idea that the bolts can be stretched by over torquing is just ludicrous. The bolts call for 30 ft/lb of torque, which produces 480 pounds of clamping force at each bolt. If you tripled that torque, and by some miracle the threads did not fail, the tension on the bolt would rise to 1440 pounds. A standard grade, Class 8.8 ISO bolt has a minimum yield strength of 92,000 psi, which on an 8 mm bolt means a tension of 7043 pounds would be required to pull the bolt beyond its elastic limit and make it permanently stretch. Even the lowest grade 2 SAE
bolt has a minimum yield of 5700 psi, and and 8mm grade two would need over 4000 pounds of tension to permanently stretch.
When the bolts snap, it's because they were loose, and were sheared by repeated impacts, or because the chain was too tight, and the sprocket folded over and broke the bolt.
By the way, grade 10 (170,000 psi) sprocket bolts are available.