Let's assume that Berardi is right, that fats+carbs do cause a synergistic release of insulin. That means that the combination produces HIGHER levels of insulin that either one does by itself. And let's assume it's also true that fat SLOWS this release of insulin.
Here are 2 factors to consider:
1) RATE of insulin elevation
2) SIZE of insulin elevation
If you eat fast-acting carbs, where the rate of increase is high, you will quickly be in fat-storing mode (unless insulin sensitivity, muscle's receptivity to storing the blood sugar, is also high and the muscles can suck up all or most of the sugar, such as post-workout).
If you slow down the rate by adding fat, you will more slowly enter fat-storing mode. But whether it happened quickly or slowly, storing fat is storing fat! And now perhaps you have the potential to store even MORE fat, because by adding fat to the meal, an even higher amount of insulin was ultimately released.
morg, it looked to me that you were considering only the effect of rate. Berardi seems to be saying, though, that a HIGH LEVEL of insulin will be fattening. This makes sense, because high amounts of insulin will drive sugar into the fat cells (unless the muscle cells are highly sensitive to it, such as post-workout).
So, you can have a fast+high insulin release (sugary drink like Coke on an empty stomach) or a slow+high (pasta alfredo); both are potentially fattening. The slow+high is maybe even more fattening 'cause the high is higher. What we are trying to achieve in our diets is the slow+low insulin effect. Just enough blood sugar to feed our muscles' limited capacity and energy needs, not so much that excess sugar gets stored as fat.