Jake,
Although it was on my Subaru, that CO figure is roughly what I saw when it failed its MOT on emissions grounds.
The car had a high flow sports cat in the centre section, but nothing in the downpipe. At the time of the MOT, it was suggested to me that the cat may not be performing to its full efficiency either through a manufacturing defect or simply because it wasn't getting hot enough being further away from the turbo than the downpipe cat would be.
Given that a Supra normally seems to pass on the centre cat alone, it may be that perhaps yours is not performing as it should in the light of that CO reading? However, having said that, I would have thought that a cat of that age would still be up to the job.