I would tend to change the gas, and a colder spark plug, but I would think your problem would lie in the injectors, like schmaydee said. If your injectors reach 100% duty cycle, there is nothing more the computer can do to put more fuel into the mixture, which would be leaning you out. The other problem here is that you never really want to be running your injectors at 100% duty cycle, rather like 70 at WOT, it gives you much more reliable fuel metering, and longer injector life.... Kindof like how shifting at 3000 rpm is better than 6000 rpm
Now this is all assuming that it has the stock turbo injectors, which, like said are good to about 12 psi, but if they are slightly clogged, that could lower your number right there to 10 or so, giving you detonation at 11 PSI. Also, check the timing and try retarding it a few degrees.
So If I remember correctly, you are looking to buy the car.... I would find out more information on it. If the owner doesn't really know, and it seems good otherwise, I'm sure you could always get the problem worked out, if you were to get a slight price break ;o)
Hope I helped,
Kris
P.S. Thinking about what you said and the gas topic... If it has been sitting for three years, and that is the same gas... definitely change it, as the octane level of gas goes down over time, and 3 year old gas isn't great, 8 year old gas could make a high compression engine not start... we've had that problem before

If you just put it in, then I would just try the highest octane you can from now on... if you didn't use it already.