I don't think RMW would go to the trouble of making that tank unless the plastic ones currently available are poor quality.  If one of those fails on a $10,000 Subaru conversion and takes the engine out, you are very motivated to prevent that.

Plastics technology is pretty simple. The best raw material is virgin plastic from a quality supplier.  The monomer (propylene for example) is linked in very long chains that are also cross linked to each other to make a strong, durable polymer (polypropylene).  Some manufacturers will use recycled plastic to save money (soda bottles and other items that are likely made from recycled plastic themselves).  Recycled plastic is chopped up or ground up and this mechanical process shortens those long chains.  Hence, recycled plastic is not as strong or durable, particularly when heated and pressurized. It shouldn't be used in critical applications.

I've had three cheap aftermarket plastic heater valves on two cars (including my Vanagon) break or leak in the past year because of this.  The OEM tanks seem to last 20 to 30 years, but I doubt these aftermarket tanks will.

Stuart