A great way to get a look at how GT oregen operates is to make a random SSP world and use /cofh clearblocks to reveal everything so you can fly around and see how it works, how large they are on average, etc.
Here's the list I use (there's a character limit for text input which is why I have to break it up into multiple):
/cofh clearblocks <username> 256 256 256 *fluid *tree *repl *stone *sand *dirt *plant *fire
/cofh clearblocks <username> 256 256 256 chisel:marble chisel:limestone BigReactors:YelloriteOre
/cofh clearblocks <username> 256 256 256 chisel:granite chisel:diorite chisel:andesite
/cofh clearblocks <username> 256 256 256 gregtech:gt.blockgranites Thaumcraft:blockCustomOre
Replace "<username>" with your in-game name, no brackets. This will reveal a 256-radius area around your location. Unfortunately the GT small ores cannot be removed or else the view of GT ore distribution would be even better. Most other blocks will be removed. Use NEI Data Dump to get mod:item names if you wish to add more blocks to the list. Vanilla minecraft items do not need to use the "Minecraft:" prefix.
More info here:
http://teamcofh.com/docs/cofh-core/features/commands/index.html