i will post a picture, strange thing is that they were burnt near the intake valve marks, and the burn is strange, i've seen detonation burns and they don't look like this, and one more thing it didn't occur in just one run, they burned in time, it is strange
now, we don't know the hp numbers, but the turbo was a k26 from a porsche, s2 exhaust manifold, s2 pistons and rods, kr crank, iridium sparkplugs, the overboost protection was set at 250 kpa, and the highest boost spike was 245kpa, it was keeping a steady 240kpa.
I'm not at the first turbo project either, and this one is the second motor to give up on us after a turbo G60 which was melted because of the driver (he didn't wait for a proper tuning and went driving at 1.2 bar with an untuned car, detonation occured and he melted 2 pistons because he kept the throttle down even if the motor was pinging as hell)
well i've seen those pistons from the G60 and d*** they were nuked bad... but the s2 ones ... it is another story, the burns are identical on all of them, and in the same spot... you'll see them in the pictures.
we have two boosted abf engines, one is with stock pistons and a 2E forged crank and rods, 0.9 bar boost on a mk1 golf gti, turbonetics t3 super 60 turbo, turbosmart wg and hks blow off valve, and that car is a blast to drive, no problems what so ever, and another one is on a mk2 golf, 1.2 bar, je pistons, and other funky stuff, and no problems with this one either.
back on topic, the abf with melted s2 pistons had the same problem as you described with distributor based spark, we checked the timing with the strobe lamp and it was ok, the only strange setting was the trigger angle which was 72.