I heard that the B packs have improved internal cooling as compared to the A packs. That is why they can supercharge at 120 vs. 90 kW. No idea about C and D pack differences.
I thought we had determined that it was the higher C rate cells that Panasonic started shipping early 2012? It seemed to make the most sense to me. If it were a cooling module, then it would likely be retrofittable even if it were contained in the pack itself. Reason is that Tesla can easily open the pack and replace cells at the sheet level if need be. Seems like it wouldn't be too difficult to get in there a replace a cooling module.
One easy way to test this theory would be to take an A pack and B pack side by side at a track. If the B pack can sustain higher power draws before being limited, then it very likely could be a cooling issue.