plastic doesn't conduct heat afaik... certaintly doesn't conduct as much as aluminium, that's for sure. The type of plastic they used is supposed to withstand engine bay heat, that's the whole point... otherwise how could they market this type of product?
Since there's been other cases of this type of failure, this is a classic case of their product failing. The pics of another product doing exactly what mine has done, were of a non painted product, further strenghtening my case. Also, as they have been posting on my350z since the beginning, and saw a large amount of their plenums being painted, if they had received a lot of complaints due to them melting (which they haven't, it's a rare issue) they should have started putting warnings in their instruction packs NOT TO PAINT. Which they haven't. And mine is a recent version of the plenum, just 1 version behind the latest.
To be honest, their product on paper is the perfect plenum: retains the oem strut bar, retains the engine cover, saves on quite a lot of weight, adds HP etc. And up to now I was very happy with the product.