This is my workaround for this—it's not ideal. Duplicate your region to a new track, or, if it's a midi region, bounce it in place. Then, also duplicate your new region next to itself, so, it's twice as long. Then, choose flex time's "Speed (FX)" on the new track with your first duplicated region, and drag from the upper-right of the region so it stretches out to match the length of the two side-by-side original regions on the original track (whew!).
I have no idea why, once choosing "speed FX" Logic does not allow you to type in a specific percentage or transposition, which would make Speed FX much more usable for pitch-dependent sound design.