The 90db sensitivity rating for 1w/1m with 45 drivers is a bit misleading. This is a measure for the SPL with 1w energy dissipated for each driver. The SPL for 1w total energy dissipation does not change, that remains at 74db/1m (according to Zaph), only it is now generated by 45 drivers burning off 1/45w each. The real gain is in lower distortion and more headroom. It is more meaningful (although not very precise due to impedance variation over frequency) to use the SPL @ 2.83v/1m measure here, as 2.83v across all 45 drivers will produce around 90db/1m.

Anyway, you are right in that my number was too low, it should be around 103,5 max SPL with 20w/driver peak. That is actually less than 5db off the RD75, so this may not be so bad. More thinking required!