"It is a productive model."
Translation:Είναι ένα αποδοτικό μοντέλο.
"efficient" is very often misused for αποδοτικό. However, "efficient" is defined as: 1. achieving maximum productivity with minimum wasted effort or expense.
2. (of a person) working in a well-organized and competent way.
In other words, it describes the method by which something is done whereas, "αποδοτικο" describes the outcome of the procedure. Now in some cases either can be used depending on what you mean. As here it could be an "efficient model" one that works without wasted effort etc or it could be "one that produces the optimal outcome. Either is accepted for this translation.