I rode mine from Wisconsin to Sturgis and back covering just over 2000 miles in 6 days. Most of the run out and back was on the interstate, with speeds at 80 mph in South Dakota. I'll agree that I would have never thought about doing it without a windshield! I made the entire trip riding 2-up with my wife along. With the 2 of us and all the gear we were over 400 lbs of payload, and the bike did just fine! We would run at 80 mph for 100 miles and then stop for fuel. I can tell you that at that speed, your fuel mileage drops off a lot! I would get 40 to 42 on those runs, but when we were just cruising the scenic areas like Needles Highway and the Badlands, I was back up in the low 50s.
As for running with the big bikes, don't sweat it! Sure, they will leave the lights lots quicker, but I didn't feel at all out of place running with a Road King and a 1300 Venture.