I don't plan on touching the diff gearing until I open the trans and put in the 1LE gear set. This assumes it doesn't explode on me. Its far better to shift less. I wouldn't go past 3.91 if you decide to do something. Best price and strength too. I've been 135-140ish in 4th gear with stock gearing, don't remember the rpm though.
I'll be putting money in wheels/tires and durability mods first (radiator, oil cooler, etc)