Theres a misconception about stretch bolts, there are stretch bolts which are aneeled (heat process) this makes them crunchy on the outside (to help roll the threads) and squidgy on the inside, theyre about 15 quid each and have special markings on them, they also stretch prior to ultimate load. Normally found in aerospace applications and connecting rods.
VW Bolts which are referred to as stretch bolts are a lower class of tensile strength which are in essence over torqued one time only, many of the suspension and subframe bolts are stretch bolts which are 8.9 class but normally get 12.9 torque settings plus a quarter turn, cheap and wont come loose - just dont reuse them