I get the aesthetics you're going for, but the matte finish looks odd. No harm in trying, though.
Personally, the taillights on the MKIII are so large that any mod only serves to bring more attention to them, IMHO.
My biggest concern would be for safety. Never mind how bright the LED's are at night- have someone sit in the car on a nice bright sunny day & walk 50 yards back & have them step on the brakes. Many, many smoked, tinted or otherwise modded dark taillights are WAY hard to see during the day, even if they appear to work fine at night. Remember- the test isn't "can I see them?" It's "do they jump out and grab my attention?"