According to the brochure, the only paper tray that supports 300gsm is the manual bypass tray, and it won't automatically duplex it.
http://downloads.kmbs.us/books/irnk/#p=2
As it is at the end of the supported specification, my experience with other equipment suggests that sometimes quality, registration, and performance may not be optimal at the end of its range. As this is an office level printer, that supports what I am saying as well...though technically it can run 300gsm, its really designed to run plain paper more than anything. The customer expectations document should outline these limitations more specifically. I have to doubt that damaging the machine would actually occur, unless you are doing something deliberately abusive. If you are attempting to run within specification, and the technicians disagree with the spec, I would consult your sales rep.
