Those FSD computer comments by Elon have been widely misinterpreted.
What he said,
exactly, was:
Elon Musk on Twitter
"Retrofits will start when our software is able to take meaningful advantage of the Tesla FSD computer, which is an order of magnitude more capable. For now,
it’s slightly disadvantageous to have Tesla FSD computer, as our software is more refined for HW2."
My guess:
Right now the FSD computer is probably only ~4% utilized and power draw is probably slightly higher than that of HW 2.5. Maybe calculation performance of the existing networks is slightly slower as well.
The HW 2.5 GPU is maybe 90% utilized during peak load.
Once they have the new, much larger networks, the FSD computer will probably be ~80% utilized during peak loads. The HW 2.5
simply won't be able to run those networks at all: it requires ~1800% of the current computing capacity, which is simply not there. Performance would drop from the required 100 fps processing to maybe 5 fps processing speed.
Elon is completely right that networks that fit on HW 2.5 can be run on HW 3 as well but it's "slightly" slower or slightly less power efficient.
But with the new networks there will be no contest.