How much damage would there be from a 7.5 to 8.0 quake on the San Andreas fault

Short answer: A lot!

Longer answer: It depends on where along the fault it occurs. If it happened in a more populated area, it would cause much damage and heavy loss of life. If it occurred in a less-populated area, there would still be considerable damage and loss of life, but it wouldn't be nearly as bad as the same strength in a heavily populated area such as San Fransisco.

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If it happens near the surface and near a populated area some very serious damage. If the focus of the earthquake is located several miles down in the ground much less. Also if it happens away from a populated area there would be less damage. Some years back there was a big 7.0 quake that happened out in the desert over 70 miles east of Los Angles that barely did any damage to populated areas.