great gridding, i did it almost the same in my biggest area's.
One main difference: i found out that gridding 9x9 works best. If u do the math on area waste:
6x6: 36 tiles to build on, 28 tiles road
9x9: 72 tiles to build on, 40 tiles road, 9 tiles 'empty' (not reachable by car)
In comparison, you save tiles when using 9x9, and, assuming you use only avenues and build a good transportation network (also with a few highways, i tend to build a highway every 3 or 4 9x9 blocks), its no problem at all.
The empty tile, 3x3, in the middle, can be used for large parks or large plazas, also good for destroying those demand caps ;-) THis way, i build quite a large city....