We have tested just about every header and exhaust combination available in the market on our in-house dynojet.
The 2 we prefer are the Belangers and CDI headers. They seem to make the best power and fit well for most year Vipers. I used the Belangers on my 98 GTS and the CDI headers on the LETHAL 650 2001 ACR. Our testing has shown the 2 headers to be comparable on a relatively stock engine. As a result, we use the Belanger for this setup as the installation is easier and they cost less. On more modified engines (heads, cam ,etc..), our initial testing has found the CDI headers to have a slight edge so we use them. However, the differences are not drastic. If someone owns the Belanger headers, keep them; I would never recommend that you swap them out for the others. There are also noticeable differences between Viper years and other mods can effect the numbers as well. We usually keep both systems in stock.
Since we always use the same car for testing and try to keep the conditions as consistent as possible (outside temp, humidity, water temp, etc..), we feel comfortable with our results.
We have around 50 Vipers in our database, so if someone wants info and results about modifications, we are happy to help.
Thanks.
-Wayne (LTHL VPR)