IF does not necessarily have to be restrictive in calories. There are plenty of benefits of doing an 18-20 fast then still eating your normal calories during the feeding window.
One of the main benefits is autophagy, or the catabolism of degraded or dead cells. Most of the stuff I've seen is anecdotal, but from personal experience, I can buy it.
Another way to look at it is that even if your consumption of calories is lower, i.e., 1600 vs. 2500/day need, your body makes up the difference in needs by drawing on your bodyfat stores (which is what most of us want!) This works particularly well if you are fat adapted. So although you are consuming less calories, your body still gets all its energy needs through the combination of dietary intake and stored energy (bodyfat). That's exactly the way nature designed it, but the key is to eat a Paleo diet, so your body isn't crashing because you aren't shoving Ho-Hos in your face every three hours.
Just my take. I IF everyday and eat about 1600 calories, 60-65% from fat, and have lost about 30 lbs in 3 1/2 months.