Tamiya rattle cans are a synthetic lacquer. I use them on occasion, but haven't decanted any, and would only do so using a respirator when no-one else is around. They smell atrocious and I can only presume have some pretty nasty chemicals.
As for your question Jesse, I can answer that!
Spraying thinned paint isn't decanting, per se, its painting! Decanting is pouring or moving a liquid from one container to another. The dangers of decanting a pressurized container I would think would be apparent. It isn’t the paints that are the problem; it’s the propellants, most of which are flammable and explosive. Mucking about with explosive flammables under pressure as a hobby is not recommended.
People do, of course, and each of us is responsible for our own actions. I don’t personally decant paint, but then I only spray acrylics through my airbrushes, and therefore have no need to decant paint.
As for the headaches, you are obviously reacting to the chemicals in the paint and/or thinners. You may even be allergic to the paints, and the smallest trace amounts can be play havoc with your body. I would question if the respirator is fitting correctly and has working filters. Is the spray booth a home built, or store bought? Improperly set up booths can force the paint fumes back into the house, rather than out the vent.