If the rear needs a camber alignment then the adjustable camber link would be the prefered method. Installation would be an easy DIY job with alignment afterwards.
The method of ovaling the subframe holes would cost just as much if done by a professional and result in a much less strong platform and a narrower track width.
Since the normal reason for doing any of this is big tires and lowered, stiffer suspension, weakening the attachment point would not be a performance upgrade.
Bosch Master, ASE Master, L1
33 years MB technician