[RarlyL8]

I am familiar with your engine setup and the gearing issues.

With the HFS you should see 1.0bar by 2800rpm. The torque curve and boost threshold of a 400whp engine using that turbo will complement taller gearing.
I have a custom 4th and 5th gear in my 915 for that specific setup.
The new gears allow the car to drop to 2600rpm @70mph. During a hard run through the gears the velocity and torque carry the engine through the final gear change right in the meat of the torque curve. Car pulls like a freight train.