There have been several articles & publications over the years that have refered to such a variation on the Firebird Carb.

Hot Rod Magazine- 3/68
"...in order to produce the advertised horsepower, there is a small tab on the throttle shaft which actuates the secondaries, but only to two-thirds open at full-throttle. Somehow you don't feel guilty at bending it rearward."

Special Interest Autos- 10/86
"..The 400 was detuned to 335bhp by adding a small metal tab in the throttle linkage which slightly limited travel of the secondaries. Needless to say, very few of these tabs remain today where GM put them."

Motor Trend- 12/91
"...GM only agreed to install the 400 engine if it could restrict it's performance, which meant it placed a stop on the throttle linkage that prevented wide-open throttle. Of course, that could be removed by the owner in about 30 seconds." (Jim Wangers)

The Fabulous Firebird- M.Lamm- 1979
"...Yet to stay within GMs horsepower ruling (1hp for every 10lbs), Pontiac not only derated the Firebird 400 V-8 to 325 bhp but also modified it so, in showroom form, it truly wouldn't produce more than 325 bhp.... by simply changing the link between the primary and secondary barrels of the Rochester Q-Jet carb. This link was arranged with a steel tab that didn't let the secondaries open more than 90% at full throttle."

Firebird Decoding Guide- T. DeMauro- 1997
"A throttle linkage restrictor installed on all Firebird 400s stopped the rear two barrels of the Quadrajet from opening allthe way, thus limiting horsepower and keeping the car within the 10-lbs to 1 hp Gm corporate edict."

So, as you can see, there was obviously something different between a GTO carb and a Firebird. Surely all of these articles were not simply created from simple rumors or suspicions but from experience. Many references were based on modifications by Royal Pontiac during prep for an article (to squeeze out more performance). No mention was ever made about the difference in the exhaust system. If anything, the HO and Ram Air Firebirds had a better exhaust system due to to Long Branch Exhaust Manifolds.

As for defining the actual component on the carb. and how it was different, there needs to be a photo comparision to better explain this. The above excerpts seem to refer to the same thing; a tab on or a part of the throttle linkage.