But like I have said before, Jason has not nerfed oregen. In fact, I am pretty sure he extended tin generation to go a bit deeper than a stock GregTech experience. Otherwise the oregen is completely Greg's thing. I suppose Jason could increase certain oregen to whatever level will satisfy you, but I would bet the exact coordinates of my next tin vein that most of the players who play this modpack would be sorely disappointed in how easy the game would be. You could resort to using Thaumcraft to make all the tin you need. Personally, I have not used Thaumcraft to make tin, but I do know that I could if I really needed to. Otherwise, again since you are playing on Single Player, you can edit the oregen in the configs. I believe the following is what you are looking for in config/GregTech/WorldGeneration.cfg. I think all you have to do is increase "RandomWeight_50=50" to "RandomWeight_50=150" or so. That should make it a little more common than Iron veins in new chunks, or in a new world.
EDIT: I bet you could also change "Nether_false=false" to "Nether_false=true" in order to generate tin in the Nether.
Code:
cassiterite {
B:Asteroid_true=true
I:Density_5=5
B:EndAsteroid_true=true
B:Mars_true=true
I:MaxHeight_120=120
I:MinHeight_40=40
B:Moon_true=true
B:Nether_false=false
I:OrePrimaryLayer_57=57
I:OreSecondaryLayer_57=57
I:OreSporadiclyInbetween_824=824
I:OreSporaticlyAround_57=57
B:Overworld_true=true
I:RandomWeight_50=50
I:Size_24=24
B:TheEnd_true=true
}