320kts to me (non-Airbii) seems a high speed for SE cruise? Like Rainboe, I would certainly not flog the remaining engine.
For interest, The BAe Lightning was more 'efficient' on one due to the engine operating in a more efficient RPM band, and it was not unknown for those who got themselves on a teeny weeny bit longer piece of string away from base than ideal on a low level detail to 'recover' single-engined.