I don't know if this is asking for too much, so I'll just put the question in a bottle and see where it goes. But I've always wondered if offense / defense comparisons are best approximated by the difference or by the ratio.
On the difference model: 10 IS vs. 5 ID would generate approximately similar shots to 20 IS vs. 15 ID
On the ratio model: 10 IS vs. 5 ID would generate approximately similar shots to 20 IS vs. 10 ID
If it's closer to the ratio, then your point 3 is compatible with Marin's more "pessimistic" comment.