IMO. 7:1 - 7.5:1 with 0.9 - 1.0 bar of boost is about ideal for a street turbo. That strikes the balance between really usable power (on the street) and engine longevity, further, you can dial in the correct fuel and timing without significant modifications (other than euro CIS) Much higher than that and you are committed to an engine with a shorter lifespan.
8:1 or higher usually requires some mixture adjustment as well as a timing adjustment. Some sort of crank-fire ignition system where the timing is much more adjustable is recommended.
The race cars: 935, 962, etc are not really good benchmarks. They ran much higher boost, much higher octane fuel, and the engines needed to last only one race before overhaul.
Es geht nicht darum wie schnell man faehrt, sondern wie gut man schnell fahren kann.
Ihr Brunnen der nutzlosen Porsche Information