What carbs and jet sizes? The real question is what speed? Mid 40's on the highway would have to be what I call damn slow and steady. My all time record with my R100 is 42mpg and I run 130 mains in my 38mm Dell's with no pumps. 42mpg is better than I ever got with Bings but I don't think I had ever ridden that slow before. I was riding almost the speed limit that day after getting two speeding tickets on one trip. I normally get mid thirties on road trips, upper twenties around home and lower twenties if I top off after a hard ride in the twisties. It all depends.
Bings are good carbs and very easy to tune. IMO they are way easier to tune than Mikuni's or Dellorto's mainly for fewer options. Mikuni flat slides? Most I have seen have independent floats and, just like Bing's "alcohol proof" independent floats, they are very problematic. I think anybody that fixed Bings by getting Mikuni's or Dell's were really fixing their own problems, not Bings. Bing's do not have inherent problems. Sure, Bings don't perform as well pretty much over the entire rev range or get quite as good mileage as well set up Mikuni's or Dellorto's but other than that they are great carbs.