You'll be surprised how fast you can burn a piston that way if you're spending a lot of time in the 4000-4500 RPM zone. Generally engines won't ping all the time (unless the timing is altogether wrong!), but if they're going to do it, they'll do it around the peak torque engine speed under open throttle conditions (when cylinder pressures are the highest). Increasing temperature will compound the problem. Lots of carbon in the cylinders will also add to the problem.
I'd set the timing back to stock, get good fuel and add a note to the winter to-do list to pull the heads and inspect the pistons. You don't want to wait until there's a hole in the piston since there's a good chance that you'll mess up a cylinder as well.
Ask me how I know...
(Photograph of a piston that I burnt myself in my ITB engine.
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