..if you went up in diameteron your front tire, the speedo will read slower, but closer to actual speed than before...
..a greater diameter rear tire will have a bit more mass..
..the loss in what you see in top speed may be related to the front tire...not an actual loss, but just an indicated loss...
..the loss of take-off speed may require a bit lighter rollers, to get the added mass of the rear tire moving faster..
..i'm purely speculating....
peace