a efficient Doff gives you +7.5 efficience rating for a total of 37.5

So after the formula efficience/300 x (75-Power level) a crew of 5 efficient officers gives you:

+6,25 in any system with Power level 25 and
+3,125 in any system with Power level 50

Given the regular setup of 100/50/25/25 a crew of 5 efficient Boffs gives you +15.625 (probably rounded to 15.6) in Power.

so, yeah, they are worth it