The Michelin Defender is not quite as fuel efficient as the OE 15" tires on the GenIII. I'm not sure how they compare to the Integrity on the GenII but I'm sure they are not more fuel efficient. It seems most people lose a few mpg when switching to the Defender BUT losing a few mpg is typical when you swap a worn tire for almost any new tire unless the new tire is the Energy Saver A/S or Ecopia EP422.
Tire Tech Information - Tire Rolling Resistance Part 3: Changes to Expect When Switching from Worn-Out to New Tires
The Defender is a good tire. It's just not the most fuel efficient choice. If it truly lasts 90,000 miles then some of that inefficiency will be made up for compared to the Energy Saver A/S which is rated for 65,000 miles. The Energy Saver A/S is good for 3+mpg more than the Defender when equipped on a Prius.