I've run a front carbon fender in the past, and will again in the future. If there was any additional front end flex, I couldn't feel it, at any speed. Keep in mind we have, the biggest USD forks ever made, and the befiest factory triple tree's I'm aware of made by anyone.
Now that I really think about it, I've never had any chassis flex that bothered me in any way on the R3, except that motion generated by a too loose steering bearing. Even cranked over, power on in 1st, hovering the front tire it stayed solid and predictable.