Surely, by moving the wheels further out from the body, even just 12mm, will cause an increased bending moment (Bending moment = Force * Distance) and thus change the camber of the tire? I'd have thought it'd put an increased load on the bearings too. I'd be wary.
In terms of stresses within the suspension, using a spacer is no different to fitting wheels with a bigger offset.
As long as you are fitting bolt on spacers then the stress on the original wheel studs is also the same as if you are changing the wheel offset.
(Though if you fit slip-on spacers with longer wheel studs then this does noticibly increse the bending moment on the stud).
The increase in bending moment on the suspension and the wheel studs through changing offset from 51 to 39 would be negligible, and not worth worrying about.
Ever checked out highly modified road cars at a race track, and noticed cars with big wheels, covered by wheel arch extensions. You may be surprised how many are still running original strut and stub assemblies - and they don't break very often. That's under the stresses of racing.
I run a modified nissan race car, running 25mm bolt-on spacers at the rear and 10mm slip on spacers at the front with extended wheel studs.
With semi-slick tyres, and pushing 213 RWHP from 2 litres (non-turbo), it generates some serious speed. It also gets bounced off the trackside curbs, sufficent to have air under one side of the car. Far more stress than most road cars will ever experience. In 18 years of racing I have only ever broken the outer flange on one axle, and have never broken a front stub axle or a wheel stud.
The point is, any increase in stress is not significant enough to be a problem on the road.
If a 12mm change in offset causes a change in camber which can be measured then you must have defective suspension components.
