It's hard to imagine the T28 being ready in time without some major change that would pressure the US Army into demanding such a weapon as the T28 by 1944. Look at the trouble the ordnance had convincing the units in the field of a need for a more heavily gunned Sherman.
Further, for the vehicle to see service, the army T28 would need to address the issue of the T28 mobility--or lack thereof.
A first step for making PACCAR's monster a realistic vehicle would be installing the Ford 775 h.p. GAC V-12 in lieu of the Ford 525 h.p. GAA V-8. This might allow the beast to rocket along at 11 m.p.h. instead of topping out at 8 m.p.h.--assuming the transmission could handle the extra power. More importantly, the increase in power would greatly increase its ability to handle obstacles.
Weight, of course, is the biggest impediment. It's hard to imagine how the U.S. would transport the 195,000 pound vehicle to the Siegfried. Special rail cars would be needed. I imagine that some sort reinforcing of the rail bridges would also be needed. I supposed some sort dual tractor version of the Dragon Wagon with some sort of a special trailer/semi-trailer could be constructed. Still even with extra wheels, I would think few roads in France, Germany, and Japan could support such a load.
Assuming the T28 could be transported and placed in position, the T28 would be effective weapon in certain situations, such as attacked fixed fortifications under fire. The gun was very powerful and the armor very thick. With a better engine, it would be a more practical weapon than the Maus--though this a pretty low bar.
Given the combined arms model the U.S. employed, the disadvantages of the t28's limited mobility would appear to exceed the advantages of its firepower and protection. Most of what it could do could be accomplished by employing the
M12 Gun Motor Carriage or a similar weapon with proper support when a "door knocker"was needed to destroy a strong point with direct fire. And again, if a weapon like the T-28 was needed, but SU-152 demonstrated there was cheaper way to do it.
Still, should the U.S. want an armored assault weapon and we are using actual US designs, then the
T29 and its brethren, the T30 and T34 seem more practical than T28. The Chieftains hatch as a two part series on this appropriately titled as
How Suitable Was the T29, Part 1 and
How Suitable Was the T29, Part 2, as well as
this entry on testing the T29 and T30.
What If U.S. T-28 Super Heavy Tank Deployed In 1944
https://en.wikipedia.org/wiki/T28_Super_Heavy_Tank
Bit of a long shot but interesting. So the United States recognizes earlier the need for a heavy self propelled gun and by summer 1944 has deployed the T-28 in significant numbers in both theaters of war. How would the T-28 preform in Normandy and against the Siegfried Line? How would it have fared in the Pacific and would it have preformed well during an Invasion of Japan scenario?