Maybe I am not fully understanding the issue, but 10 MP per 100m elevation is basically a standard amount for all FWWC, PzC and MC games since the beginning. It was a value that was decided to be sufficient enough to not be considered insignificant and it has been in use for some 15 years or so.
Also, your comparing road movement in FWWC to elevation penalty and this is not directly comparable in distance. If you have ever tried marching up 100m slope over a distance of 1000m (maybe you have!) then it will slow your forward progress. That is what the elevation modifier is for, its not a literal cost to move up 100m in height, it is simply the extra effort spent moving up that elevation that penalizes the overall distance the unit moves. Keep in mind that the elevation change in the game doesn't mean the units are stepping up an abrupt elevation change, instead on the actual map it usually means a slope across the full distance of the 1km area. So, if you march up elevation change of 100m over the distance of a 1000m hex, then I imagine that this would likely slow your overall progress by ~1000m overall. It is not unreasonable to me at least.
All that said, sure I suppose someone could argue that 10 MP / 100m elevation is too much all these years, and you can change it to 5 MPs if you like. That said, since it is has been this way all this time then I am inclined to leave it.
Regarding road speed, in the FWWC notes I explained (or I thought I explained) that primary road movement for foot units basically costs less over what it is in other games, in order to represent historical marching distances. Now this difference in the series I guess this could be justified that WW1 infantry generally had less equipment than your WW2 or MC infantry.
Johnny, not sure I understand what you mean about it taking 2 hours to go up 150 feet elevation, it only takes a portion of your MPs to do that, not your full MPs.