Sorry if this is to techinical but this is how this works:
It has to do with your Longitudinal center of gravity. Having the aft tank full which is the aux, will change the performance characteristics of the boat. The boat will "dig a hole" on plane and burn more fuel. Even trim tabs will not fix this, yes tabs will fix the trim angle, but the boat will still be lower in the water since the tabs will apply the same amount of lift force regardless of where weight is. This means it may run at the correct angle, but it will not push the transom of the boat out of the water to run how it was designed to, or at the boats optimal location. For fuel efficiency you want the boat out of the water as much as possible, the less wetted surface area, the less drag. Also, weight forward is better for ride, the boat is heavy aft from the engine weight. Just like inboard boats, they ride better because the bulk of the weight is located over the boats center of gravity(near the center of the boat). Having the bulk of your fuel there will benefit you in ride and fuel. With the bulk of your fuel aft, you will have a lot more bow rise when you run into a head sea, having the weight forward will help the boat cut the wave, this is a deep v boat and is designed to run and cut waves. Grady White designates their tanks main and aux for a reason, if there was not a reason, they would call them tank 1 and tank 2.
Grog: As for a better ride, that is a toss up. On a smaller light boat like that, having the aft tank full will help to keep that boat in the water in bigger seas. Since the boat is light, weight aft will keep the engines in the water and it may seem like a better ride since they are not flying out of the water and pounding. Doing so will also cause the bow to rise and fall more in larger seas. This is a trade off and on a small light boat like a 204 it may be a good one. As boats get heavier and bigger, moving weight and fuel towards the center of the boat is ideal. Larger boats do not fly out of the water as much(but they still do), so this is less of an issue, and now you want to target the bow rise and fall. Eliminating this will allow the boat to cut through and smooth waves instead of riding over them and pounding.
Hope this all helps.
Happy Holidays!!!