Due the new map generation, I have a huge line of deep ocean all around my generated chunks...If you started your world before version 0.5.1 then the Mega Taiga and Savanna biomes should be fixed and no longer spawn. It is why I recommended to restart a world if you spawned in these biomes as I intended to fix them.
Restarting seems very rash. If you just wanted a harder map why not move your stuff to a tainted biome?
I will just do some 'witchery voodoo'(a.k.a NBT hacking) to create a new map and keep all my current progress and items.
But before that, I am just checking some stuff that I noticed on my old world... If I am right, you might have a small problem with the taint...
What is the taint life cycle? Terrain gets tainted, then fibrous taint grows on the block, it matures until converting the block into crusted taint and then the crusted taint dissolves into slimes or liquid goo?