Timing and boost, which is governed by fuel. The more of each the more power. Ford big turbo can be pushed past 400 on 98. But if you want more may as well go E85. BUT it you want to stay with 98 just get a bigger turbo.
Here is 400rwkw on 98 at 15psi, factory BF F6 turbo. We could push this past 20psi on this engine, reduce timing, but lower boost is better.
You probably want to stay at about 18psi for unopened engine.