I say its BS and esp when its a vendor who has a vested interest in the results and how customers then do free marketng of that vendor
The C5 really did not have restrictive exhaust and C6 even less.
To get 30 at the rear wheels with about a 14% loss over drivetrain I do not buy their numbers.
Positive gain can really only be seen if the engine itself is constrained by the manifolds and being the CATs on a C6 are right to the manifolds I highly doubt the gains plus the fact to do LT headers on a C6 means you have to move the CATs now back at least 2 feet from stock location so a tune of the PCM would be a must. Also means the new O2s that are close to heads are also moved rearward so I just do not buy LG claims at all.
Being the CATs now hang off the heads it will be real easy for a smog shop to tell if they have been moved or taken out where at least on C6 they would have to get under the car and they are too lazy to do that in many cases.