First, we cover two of the most powerful steam locomotives in the world. Then specifications (boiler pressure/drive wheel size/firebox size/counterbalancing/service records). Then compare the american system and it's achievements to everything else. America may not have invented everything useful in steam technology, but they sure came close to perfecting it.
I find it very hard to believe that not even ONE american steam engine could have (or actually did) beat Mallard's record. The specifications between Mallard and other american steam engines are similar (boiler pressure/drive wheel size/counterbalancing/firebox size). If only I could actually speak to someone credible in person. A lot of the information out there is rather anecdotal.
In order for Mallard's record to survive, something would have to go wrong with the PRR 5550 project. Which is unlikely, as the T-1 trust is very clever, and would not be spending this much time and money if there was any uncertainty about the success of this project. They purchased a tender, a coal stoker was donated, and things like this really help save time and money.
I do not speak for the T-1 trust, but have donated many times in the past.