In the realm of OE production engines, the altered firing order is said to increase engine smoothness. In fact, when we questioned Gen III valvetrain engineer Steve Pass, he said the firing order was dictated by the crank guy. That was Bill Compton, who had this to say via e-mail: "Since the G3 engine was a clean-slate project, each group looked at things that they could change to optimize the performance of the small-block. Although we did not have an issue with crankshaft loading on the older V-8s, there was room for improvement in the area of distributing the peak firing loads among the five crankshaft journals. Analysis showed that main 4 had peak loads significantly higher than main 2. By changing the firing order, the peak loading on main 4 was reduced and the peak loading on main 2 went up. Overall, the loading through the mains was much better balanced. By improving the load balance across the crank, we created a better balanced oil film interface across the crank. The valvetrain group simply changed the cam lobe timing to work with the new crank firing order."

I'd venture a guess that... the larger the plenum the less relevant the firing order.