A: No, Sphero will move slower because of the nubby cover.
I really love the nubby cover, don't get me wrong, but I would so love you guys to put out a simple rubber nubby cover as well... an additional version sans the pimples. Same rubber and same (or new) set of colors. This first pimpled one would be for off-roading (swimming, gravel, etc), and the smoother one would be for speed and precision. Both would provide better traction and protection.
I agree with John's comment. I would love an additional nubby cover that is for flat surface speed and helps with smooth surface turning without being so bumpy and slow. The current nubby cover is great for rougher play and outdoors.
Great news! Turbo covers are coming soon, watch for them before the holidays!