The biggest limiting factor to train speed is wind resistance. The horsepower required to overcome aerodynamic drag is proportional to the SQUARE of the velocity. Downhill, with a tailwind, a lot of things might be possible. The record-holding Mallard had a fairing that was designed in a wind tunnel, and it set the 126 mph world record on a slight downhill run. The German loco which held the previous record of 124 mph was also streamlined. This might be one case where that cowl wasn't just for looks. In the years just before WWII, the pride of England and Germany were at stake! Even so, neither of these locos ever claimed more than 4000 HP.
A couple of other considerations... Above a certain speed an unbalanced machine will shake itself apart. The valve events and exhaust passages have limits too. The K-4 just wasn't as advanced as an FEF, Niagara, or Milwaukee F7 Hudson in these respects.
Now, the T1 duplex had precision valve gear and a streamlined shroud. It was dyno tested at 6500 HP, much more than Mallard or the German class 05. I would believe the T1 could break 130 mph before I would believe the K4 could break 110, especially pulling tonnage on the level. I'm not sure why the Pennsy never attempted a record with the T1; they were built after WWII and I guess by then no one cared about steam technology 
. Since there is a group of enthusiasts who are trying to build a 'new' T1 based on the old design, I guess it's remotely possible that we could find whether it's really a record-breaker. Doubtful, but I never thought we'd see an operational Big Boy either!!
