I too have wondered about the gap in serialization from 75xx - to 8999.
I suggest another possibility for you to consider. Could some cars rebuilt by the Factory have received ~new VINs for some reason ?
For a few cars in the 8000 range, they have a common 'history'. They were returned to the Factory for significant work.
Another example, we know 2263 (the first Group 4) was crashed very early in life, then sat on a pallet behind the factory for a number of years. Later the car was 'rebuilt' as 8263 in GT5 trim. Recently it has re-appeared, now in it's original Group 4 trim. Details posted here in the forum. Great reading.
The red Pantera (here in the USA) was photographed by Mike Drew. At the time, Mike told me that the owner had sent the car back for extensive work. It was returned with a 8xxx VIN.
Anyway, something else to consider, and I look forward to your thoughts!