Depends on how you drive and how you rotate your tires, but I would say between 1.5X and 2X. Also note that your treadwear warranty may be shorter with a staggered setup.How much more mileage would I get with the square set being able to rotate the tires?
Yes (well, depends on what you call substantial, but certainly noticeable). That's why people do it. Just go for it.Would the bigger tires provide substantially more grip?