If they get to 20% efficiency, that would be 100% more than before.
The claim I quoted above was that algae produced "100x" more oil per acre than food crops, not "100%" more, which I agree would be 2x more. If I'm right that food crops are ~10% efficient, then it would mean algae is 1000% efficient, or 990% more than food crops are. Except that's physically impossible, so something must be wrong - me, or the claim.