Here are my thoughts and theories on this.

Don't forget that with all the cylinders tied together, any reversion is going to be worse, in other words if you install a relatively restrictive chambered style muffler, the feedback (reversion) from the single exhaust that tends to restrict the intake will be worse than if you have an isolated, dual-pipe system because now you have all 8 cylinders acting on the one that happens to be exhausting at the time. A higher flowing muffler will likely not hurt as much but the location of the junction may be more important.

Granted crossovers tend to tie both sides together but not in the same way at the manifolds as a larger single large pipe.

I think if you keep the pipes split all the way to the back then combine them, it may act differently than if you tie them together at the front and run one single all the way. Plus with dual mufflers you can lower (quiet) the exhaust sound level even more due to the added volume of the two muffler bodies. Always remember, you want volume in the mufflers to get the most sound control. If you have multiple pipes feeding/traversing through a muffler, that takes away volume you can depend on for noise/sound absorption.