1. It won't increase thermal efficiency - so if you're putting energy into a FW, it's not going to make running the engine any more efficient
2. At steady state - there's no difference
3. You can shift earlier without dropping rpm as much
4. You'll be able to go further while engine braking
So for 3 and 4, I don't see that big of benefit... If you're going to coast, EOC or throw it in neutral.... I don't see 3 as a big deal - 99% of the time you'll be in or out of gear, not shifting...
For 4, you can get the same benefit by adding dead weight to your car...
If the FW can be disengaged while not "charging" or "discharging" that energy... That would be beneficial for city driving