Alloy pistons will melt if the plugs are too hot. It happens regularly in motorcycles (albeit a little more quickly due to the higher state of tune).
Have you had the heads off to check the pistons? In Australia 380 pistons are $450 ea.
Bosch supers are $2.90 and platinums are $25.00
Platinums last 90,000km
Supers last 20,000km

That means that for every platinum plug you have to change the supers 4.5 times.
4.5 x $2.90= $13.00

Thats half the price!!!

Oh, it does take about 4 minutes to change the plugs on the 300e.
And there is no risk of holed pistons!

Win win situation. Go with the old.

__________________
Andrew
300e 294,000mi
380sel 185,998mi
380se 309,980mi