Speed isn't that important. B2 can't pass 0.8 mach either, so I hope they optimize the twin engine variant for maximum payload.don't know how credible the account is, so take it with a grain of salt.
"An Official:
- ANKA 3 -
-Cannot exceed the speed of sound. The new version of this, a twin-engine aircraft will exceed the speed of sound.
-We made the 2nd and 3rd and are currently producing them."