Both the Carter BBS/BBD and the Holley 1920/2280 have their benefits and drawbacks. There are things to prefer about both designs, and at this late date (when one must generally take what one can find), often the biggest carb-related factor in how well and efficiently the engine runs is the condition of the carb, rather than its make or model. The BBS does generally tend to age more gracefully than the 1920 with respect to durability of metering accuracy (the 1920's metering block tends to clog up and die), but the 1920 ages better with respect to physical condition (the BBS body castings tend to warp). The Holleys are easier to re-jet, but the Carters have more thoughtful float designs. As with any carburetor, you kinda have to pick your set of compromises and go with it. It's easy when you've got the choice of a new or as-new Carter, say, versus a used/worn Holley (or vice versa). The choice gets tougher if you have the choice of a new/as-new Carter vs. a new/as-new Holley!
I get about 17 mpg in the city, but I have heard that I could be in the 20s! 
My speedo is about 3 mph off (over), I have passed many many cops and they have done nothing (knock on wood) 