If you polish it at all, it will gloss up. You need a repaint (detailer of 20 years experience here). Its the main reason ill never own a matte or satin car.
This is an interesting idea. Im concerned the bolted on carbon plate is going to look out of place. I wonder if theres a way you could countersink it (if that makes sense) so it would be the same height as the stock handle housing. Maybe shave the housing down and epoxy? Just throwing thoughts...