The gun will be FINE. I think running duster in a tm is just people being over cautious. I'd rather have TM plastic than metal anyday of the week. Plastic has many upsides compared to metal on gbb pistols.
Pro tip - Tokyo Marui gas blowback pistols DO NOT need metal slides to run propane. Trust me I own one.
Follow up - I don't suggest metal propane adapters. The one I had broke my mags in < 3 weeks. Plastic adapters have many more pros than metal ones. Don't believe me? HolyHandGrenade has the most experience on the forum (that I've heard of) with any adapters.