I prefer poly thread for my bobbins. I keep my iron on med heat and no problem with poly melting although I have never had quality poly melt when pressing seams using a hotter iron. I tried to melt a poly seam and it took much longer then I would ever leave an iron on the seam.
I mostly buy Superior poly threads and pre wound bobbins for piecing. Mettler thread is not silk wrapped, it's silk finished. No silk in the thread at all. Everything you want to know about thread is here:
http://www.superiorthreads.com/education/