If I were welding that, I would use an argon-helium gas mix and 4043 filler rod. All the surface oxidation should be removed in probably a 1/2" vicinity of the weld (in all directions including wrapping around the corners) and all bolted and pieces removed probably at least 8" away and all rubber pieces removed probably 12"-18"+ inches away. And it would be an absolute "not fun" job, TIG welding that with the auto darkening helmet on your side/back under the car, filler rod in one hand, TIG torch in another, hopefully some sort of special torch-mount amptrol or at minimum, on/off torch switch set with upslope and downslope settings of a more advanced TIG machine, as a foot pedal sure won't work in this "out of position" job. On/off switch with proper machine settings might do the trick. You will need a very experienced welder and even then, if there isn't a clear path o vision or awkwardness in the access with both hands, I'd still budget on dipping the tungsten a couple times. You'll also need to get a good ground directly onto the subframe somewhere, and there will be a fire risk, so absolute no brake lines or gas lines anywhere near.
It would be much easier to remove the part to TIG weld this. And then, the points are valid about the heat affected zone annealing the material in the vicinity of the weld. Over time, it will regain its participation hardening, but it's a concern on a loaded suspension part. Welding hot and fast (and using helium in the mix) will reduce the width of the heat affected zone, but not eliminate it. This is probably
A356 sand cast aluminum, like many other cast aluminum car parts, has silicon and magnesium alloying elements. It welds up pretty nicely actually. Since it's a suspension part, I would seriously consider passing on this job if it were to come my way back when I were doing these sorts of things. Too much liability. These sub frames used are available cheaply, so bolt on part replacement seems a much easier bet. After welding, especially if done "on car", you'll also need the weld deposit ground to proper clearance for the eccentric washer to bear against. It's possible to be done and all, but sounds like no fun to me and not something I'd involve myself with fixing when replacement sub frames are so cheaply available off pretty much any wrecked Boxster.
JB weld actually wouldn't have the problem of the heat affected zone, so could be advantageous in this regard. But if your camber adjusting bolt is moving around, I would question if JB weld is up to the job, and strongly consider just replacing.
Hope this helps. Best of luck!