Having just watched "The code" on netflix, I can offer what was done by boeing in their sims(In the 80's mind you). They used random heights, and what appeared to be midpoint displacement or another fractal function to create the fine detail. The run-time looked like ROAM, He gave some detail as to the vertex count, but i can't recall of the top of my head. You might try this to fake increased heightmap resolution when creating your patches. Or get a better data set. I'm currently learning how to stream patches myself and using the SRTM 1 arc minute data.
What do you expect the visible distance to be? knowing this would let you figure out what the max area that would need to be in memory at once. From there you could start to figure out where the trade off goes for increased number of patches ( with less LODS) vs increased patch size( with more index buffers) but my instincts tell me that 10 miles x 10 miles is large for a single patch.