The Terrain Data will be 7 times bigger too .
Not necessarily. Most Data would still be for blocks of 7. Stuff like material info and the like Terrain would be just like water, x/7. Oh, and this would allow to have less than 7/7 water freeze realistically.
So if I have 4/7 obsidian and 3/7 granite, which is on top? What about 2/7 obsidian, 3/7 granite and 2/7 obsidian? Are they the same?
A way around that would be to only allow a tile to have limited types. It could have a biome-designation, which would describe soil availability/composition, and a designation for the type of stone, which would limited to only one object.
My main issue would be displaying it. You would either need an extraordinarily complex display/mining/construction system which would be painful to use(so that you'd know which tiles were usable, and so that you'd be able to set the mining/constructions to be usable), or a full 2,5/3D interface(which would still make designations a bit obnoxious). Unless you have some ideas for that, considering the current interface of z-levels would only be able to give
useful information with a division of only one(as it is now), possibly up to three or four.
A second question of mine would be whether or not this should be expanded to give each square's x- and y-based layering as well, to be completely fair to those of us that are huge fans of these under-represented dimensions. This would result in 7^3 = 343 Intuitively Derived and Described Material Cubes(TM) per tile, giving us quite a handy tool for quickly reading a tile's most important characteristics, and allowing for more accurately represented rounded corners. Of course, one could ask if this would be too much to store in memory, but really, if you do out the math, it would only give us around 6,303,744 more bits per mapsquare per z-level. On my current map of 9s x 9s x 89-z, this would be only 45443690496 extra bits! That's only ~5.3 GB, and anyone that doesn't have at least 12GB of RAM on their Dwarf Fortress computer is an undedicated coward and should be made into mittens post haste.
Though, of course, the real use of this system would be to allow us to fit up to 342 more creatures in each tile before they have to start laying down.