I can't say as I've had any real problems with copper even in the 525F range when I've coated it with thermoplastics. If you are that concerned about it, go with your low cure schedule for longer. The biggest hurdle I see you overcoming would be adhesion in this instance. Look in the powder coating supplies for a pre-treatment for the copper. That should help keep things in check with grip, and the lower temperature (even though I've never had a problem with it in my experience) should clear things up for warpage..... hope that helps,mate
.... Russ