On a racing bike there is no top cap, it'll go until you run out of HP to make it go or max out your rpm's if you are really playing with stuff. I don't believe I have ever hit any sort of 'rev limit' on my bike so can't tell you what that value might be. I think i seen 12k motor RPM at one point somewhere though but don't quote me on that.
changing your sprockets, yes it is going to change your torque, and change your acceleration. You don't want to get too wild where you are lifting the bike up every time you turn the throttle, or do you?
On the Ribelle, you are sitting up .vs. prone on the EGO, in a race position, so your center of gravity is higher, your body weight is not holding the front end down to the road as much as it does on the EGO. Do you want to turn your bike into a wheelie machine or keep it something a bit more controllable?
As for the speedo inaccuracy I believe they fixed that in rev 43, however on that, yes, anytime you do change sprocket arrangements you will change the accuracy of the speedo since a non race bike does not have a way to put in your sprocket tooth count that I am aware of, to alter the math on it. This should be easy for you to figure out though just doing the basic math on it though.
Aaron