Let me stick my neck out at the risk of receiving some stick for the crowd here.
Not everything is measured in time and money or resale value. If we did that then we would be a rather one dimensional individual. Sometimes one does things for pleasure or joy or OCD 
. There are those who crawl under the car with a tooth brush and rag to clean the chassis/suspension where no one will ever look but gives owner immense joy from ownership. It could be a rickshaw or Rolls. As they saying goes in Urdu, "Shauk ka koi mul nahin"!!
Going back to the heat shield, I think it is a MS sheet pressing with galvanic coating. The coating has broken down allowing the MS to rust. No amount of polishing will fix it. That will result in further removing the coating and making it worse. High temp paint might last a few months. Only solution is to replace it.
