the body is already spot welded along the seams, so you would be adding weld in between the spots effectively.
I think AWDmoke is trying to say that the rigidity you need to make the suspension work is found in the subframes, so welding them up will help in that area.
what seam welding tries to achieve is a rigid mass that is suspended by the springs. This makes it more like the mathematical modelling that is used to determine suspension settings (spring rates, damping rates, roll settings etc) so the suspension becomes more tunable and behaves as predicted by the models. Is it worth it on a road car - nup. A race car - yup
but for a race car yu would gain far more rigidity by constructing and attaching a properly designed roll cage - think like a V8 supercar - some of the mini miglia racers in the UK go to this extent with fully triangulated roll cages that are attached to the body at key places to make it all stiff as hell
sorry about the ramble - had a couple of wines at lunch

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