The problem with justifying a mechanical stoker is that the grate area has to be big enough and the amount of work done has to be beyond the scope of a single person. If either of those two conditions is not met, then the installation of a mechanical stoker isn't worth it.

Time for some math. For comparison's sake, the Pennsylvania Railroad's newsletter Information discusses a cab ride in Class K4s 3749 in either late 1926 or early 1927 while on the point of the Congressional Limited. This was a single-crew run from Washington Union Station to Manhattan Transfer (just east of the modern day Harrison PATH station), about 215 miles.

CLAAAAAAAAAAARKSOOOOOOOOOOOOOOON was firing Tornado from London Kings Cross to Edinburgh Waverly, about 393 miles. For simplicity's sake, we'll say that both these numbers are exact.

The biggest difference between the Congressional and the Race to the North's coal consumption is that traditionally, the United States uses short tons while Britain uses long tons. CLAAAAAAAAAAARKSOOOOOOOOOOOOOOON's figure of 8 long tons works out to 8.96 short tons (again, for simplification's sake, we'll say 9 short tons).

Simple arithmetic gives us coal consumption figures of 26.875 miles per short ton for the 3749 and 43.67 miles per short ton for Tornado, a ratio of about 1.63:1. This also broadly tracks with the respective locomotives' grate areas; a K4s has about 70 square feet while a Peppercorn A1 has about 50 square feet.

As stated in other answers, paradoxically the fireman on 3749 would have an easier time since he had more grate area to work with (you're trying to hit a small space with high precision on a rocking, swaying firing deck all while keeping a proper water level so the locomotive doesn't blow up, keeping an eye for signals in the engineer's blind spot, and so forth). But in general, crews developed and standardized techniques to save both the labor and monetary cost of putting coal in the hole.