At the present time he has Supermemo Online and that should provide an interesting data set. But I don't think he had that dataset at the time he wrote those lines.

I think Piotr worked a lot with his own data. But he also writes:

The increase in the speed of the convergence was achieved by employing actual approximation data obtained from students who used SuperMemo 6 and/or SuperMemo 7

Algorithm SM-8 is constantly being perfected in successive releases of SuperMemo, esp. to account for newly collected repetition data, convergence data, input parameters, etc.

He also described it in his thesis in a bit of detail. 32 test subjects does not compare to the Mnemosyne dataset but it does provide plenty of data for testing algorithms and the might be enough data to decide that SM-8 is significantly better than SM-2.