I like the idea of permitting infantry units to "force march" from one location to another. Marching is a big part of moving ground forces and marching fast is highly desired in some situations. A forced march is simply another option for ground movement as is moving infantry by truck, or rail, or by some other means.
As the O/P suggests, there should be a penalty however, perhaps higher attrition/loss of equipment or something similar.
I don't see the absence or presesnce of a forced march feature being a huge issue although it would be a nice addition IMO (few of the many features of this game are "huge issues" by themselves...but the totality of the features makes for an enjoyable game).
You might find this snippet of interest. I scanned from one of David Glantz's works -- "The Initial Period of the War on the Eastern Front, 22 June -- August 1941". It's actually part of the Seminar Proceedings for the 4th Art of War Symposium and included in a paper written for the Symposium by Kenneth Macksey entitled: "The Smolensk Operation, 7 July - 7 August 1941". I highlighted the passage of interest in grey -- although there some additional context material that is of interest and included in the snippet.
30-miles a day is about 48.3 kilometers/day. Close to 5-hexes a day in game terms. Assuming you maintained this rate for 5 days of the week, that would put an infantry divisions movement rate at about 24-hexes\turn. If this pace were maintained for 6-days of the weekly turn, it equates to an Infantry units movement rate being about 29-hexes\turn (etc.)
In-game movement rates for Axis Infantry Divisions early in the 1941 campaign are typically about 10 to 15 -- occasionally you see high morale units and Jaeger and Mountain divisions sporting a movement rate of 15 to 16. But these are not hexes\turn as I have indicated above. Moreover, an infantry division’s movement rate is halved when marching through enemy controlled or impending control hexes. In-game German infantry divisions in the crucial maneuver part of the 1941 campaign are really kind of hamstrung in terms of the amount of territory they can cover in a given turn -- at least when compared with a historical march rate of 48.3 kilometers/day. When moving through enemy controlled hexes and impending control a German Infantry unit is only covering about 50km/WEEK to 75Km/WEEK vs. the ~50Km/DAY cited in the Kenneth Macksey Paper.
Doubtless this is part of the reason we see German players complaining that that can't form proper pockets in game. The Axis mechanized units end up doing almost all of the work associated with pocket formation as well as trying to keep a pocket closed because Axis infantry is typically too far in the rear to provide additional oomph to pocket formation, maintaining pocket closure, and pocket destruction. At least this has been my game observations in head-to-head when the battle moves beyond the Polish-Russian frontier and into the hinterland.
Having said all this, and having done a bit of marching in the Army as well as a fair bit of hiking, I think it pretty safe to say that 48-kilometers a day is a lot of ground to cover on foot. Therefore I reckon the 48km/day has to be reflective of a short term forced march rate of movement. But I'd argue that being able to cover these sorts of distances in-game on a short term basis would put a rather different spin on a players in game capability and how they go about planning their big pushes.
A major flaw in your analysis is that each hex equals 10 miles (not kms).
Moreover, I don't believe Macksey's figure of 30 mi/per day, except in very limited circumstances. First off, I don't think it is possible for men in full pack to march 30 miles per day for an extended period in the middle of a Russian July, especially when they are not marching from prepared position to prepared position. Second, even if the men could do it, they would not be moving as a combat ready force, because all of their logistics would be trailing far behind (as the article mentions). I actually think that the 16 mps that certain inf units get is pushing the envelope.