Ilsa, my interstate slut, has no problem doing a steady 85+ with the massive RT fairing. She does bounce around a bit when that big barn door gets caught in a crosswind. Otherwise, she is a nice solid ride.
As far as fuel economy, she gets a pretty consistent 45-48mpg on 87 with 10% pretend fuel up to about 70mph. Above that, efficiency gradually drops until she's getting around 40mpg at 85mph on 10% regular 87.
Those city cases were probably part of the police trim. A lot of the police K bikes I've seen had front and rear crash bars. The rear bars only stuck out enough to protect city cases. Now that you mention them, are the city cases painted white? Are there holes in the fairing for mounting lights?
I have seen photos of British police bikes with calibration stickers for the speedo. It would be interesting to see if your speedo reads high like other Brick speedos. You might have to be more careful if you don't have that optimistic 4-5mph "cushion".