Probably close to 9-1 if the gaskets are thinner than the stock ones, which I think they are.
Remember that 600 hp made with 20 psi puts as much strain on the engine as 600 hp made with 25 psi of boost. All the engine sees is combustion pressure in the cylinder and it takes X amount of pressure to make a given hp. The major factor is that a big turbo might flow a given amount of air at a lower temperature than a smaller one might at a higher boost required to get the same amount of air.
