Wallenstein vs. Shogun Map Comparison
Looking at these maps you can see that there are a lot of similarities. All three maps are made up of five regions with nine provinces in each region. All of them have one central region surrounded by four other regions. The Shogun maps are elongated but (for the most part) so are the provinces themselves. There are roughly 10-12 provinces spanning east to west and 5-6 north to south on the Shogun maps where there are perhaps 7-8 spanning either axis on the Wallenstein map. Almost all of the provinces border at least three other provinces on both maps and the water routes on the Shogun maps do quite a bit to make the edges more accessible to some provinces in the center.
While it does look like it's slightly easier to grab and hold a corner of the map in Shogun, it's perhaps not as much of a sure thing as it initially appears, particularly when you take into account that the more valuable provinces tend to be closer to the middle.
Furthermore, the Sun and Moon maps in Shogun offer different area distributions which add quite a bit of interest. On the Moon map in particular (the bottom one), the areas are intertwined such that it's very possible to have a presence in several political areas while remaining entrenched in only one geographical area.
So I guess the conclusion I draw from this is that if you were holding off on getting Shogun because of concerns with the maps, don't. The maps may be different from Wallenstein and they may offer different challenges but I don't believe them to be unbalanced.