Honcho, here is my .02.
I have heard people discuss that their bumpers don't match and call it fading but I am skeptical on that sort of explanation. Understand, however, that I am not near as knowledgeable as you about these cars and while I have owned an NSX since 2007 my 2005 has always lived in a garage. My 2004, which I just acquired a few weeks ago, seems to have also lived a pretty nice life despite 40,000 miles. I looked at 11 separate Formula Red NSX's in my year long search and I only saw one car with a distinct bumper mismatch. Of course, two of them admittedly had the front cover painted and one of those was detectible, but acceptable, to me. I only passed on that car only because we could not get together on price -- I could have lived with the paint on that one.
Before my purchase I looked very seriously at a 2005 New Formula Red car (still on Ebay) and both bumpers clearly did not match and it was far enough off that you could easily see the difference in the photos. Your car is much closer than the Ebay car. That Ebay car was on BAT prior to that and there were a lot of paint comments. It's worth studying the pictures if you are curious and, if so, you can find them easily because the car was sold by "The Dude Abides." I passed on the car because I knew that I would never be happy with the mismatch. The Seller, who had only owned the car for a few months after buying it on BAT, insisted that the bumpers simply aged differently than the rest of the car but it was too far off for me. No doubt in my mind that the Seller believes his bumpers were never painted and he is a super nice guy -- I wanted to buy his car.
In my search for Formula Red cars over the last year there was one very interesting discovery which also merits this discussion. That is, I saw a real pig which was a non-running 2003 model with 130,000 miles on it that appeared to have lived outside most of its life and the interior reflected that sort of abuse but, oddly enough, the bumpers did match the body and the car was original -- albeit faded, paint. I toyed with the idea of bidding on the project NSX for a few minutes but decided it was too far gone. I will bet that if you see any red body panels on Ebay that it came from this car because it was straight as an arrow and was purchased by that well known salvage yard in Rancho Cordova, CA that you often see on Ebay.
Finally, I can say that my 2001 Z8 and my 2005 Ford GT both have painted red bumpers and match exactly.