I would guess I run in the 5.x range as well since full low is 3.7 according to the book and that isn't fast enough for me most of the time. I definately don't typically mow in full high unless I am really in a hurry because rain coming or some other urgent appointment.
BX1500, high range about 75% throttle, working the heck out of the HST with my right foot reversing, turning around trees, etc. No more than 1 hour for 1.5 acres (not all is mowed). If it takes more than an hour it is time for a bigger mower! I go to low range only when climbing one hill in the yard.
I occasionally use the BX1500 to mow 5 acres at church. Most of it at high range, pedal to the metal, 8 mph max speed! The ground is much smoother over there. Sometimes I go crazy when I look into her eyes.... (The BX1500, that is).