How nerfed was Starkiller in the books'

I always hear about how game Starkiller is much stronger then his book counter part because it was supposed to be a power fantasy where you are the strongest like in God of War.

But the books had to tone down his accomplishments because he was just to powerful, but by how much did he get nerfed?

Did he go from Vader level power to Mace Windu level of power or was it more drastic?