I think the 18x9.5, 265 square setup has a small advantage due to the increased front end bite and the smaller diameter tires which provide a slight gearing advantage.
As implied in your post, the Re71r's themselves, are the greatest speed adder from the stock tire set up, so either wheels, staggered or square, are going to massively benefit from the Re71r switch!
Even though we are just talking which is faster here, I cannot help but mention the HUGE benefit of rotation with the square setup as I like to get the most out the the expensive track tires I seem to constantly buy year after year!
