One thing that I do in my personal modpacks is install Underground Biomes Construct (http://www.minecraftforum.net/forums/mapping-and-modding/minecraft-mods/1293503-underground-biomes-constructs) and modify the configs so that ores will spawn in the same level as the corresponding real life rock types. This would be a neat idea if the ores would detect different 1.9+ rock types during spawn and place ore clusters near their associated rock types.
Im assuming with my lack of minecraft code knowledge that this would be really difficult with vanilla rock variants although I did have a suggestion to remedy this. The mod Underground Biomes Construct has already underwent one owner transfer and the current one hasn't updated the mod for quite some time. What if you acquired the code in that mod and incorporated it into the next Metallurgy iteration and turn it into a whole underground spawn overhaul mod? UBC seems to be fairly stable and already has texture support for all metallurgy metals in all 24 rock types. Going one step further you could limit what rock types spawn in which above ground biomes with each ore following the same pattern.