I really can't explain the mechanism where using an effect to scale clip content always crops at the frame edges of the sized clip and the Transform Scale scaling everything as you would expect. The gist of it is: Transforms are at the *end* of the "Effects chain" and *everything* is scaled. Effects applied in between the "clip" and its Transforms use a different mechanism. By using this effect, you are trading that ability (of Transform) for the easing (you can still accomplish the same thing, it just takes a little more work.)
The curvature of easing is fixed. It only goes between 0 and 100% and it really isn't significantly different than the built in easings in FCPX.
The effect of easing is more noticeable the more time you can give it (and I mean real time, not just a percentage of the clip - longer clips with more time can be made to have more easing applied.)
That said, it doesn't mean you can't cheat! If you apply one Scale All by Clip Length effect and set the timing for 1/2 the length of the animation you want and ADD ANOTHER Scale All by Clip Length and set the animation for 2/3rds the length of the animation you can kind of "double" the effect of easing. I'll give you a settings example:
Set Scale All (1) with a Start value of 0% (or your minimum); End value of 100% (not more - you have to "reset" the quality to be 100%) - Curvature = 100. Start Time = 0, End Time = 25%.
Add another Scale All and set the Start value at 100%, End value at your Scale Max (say 300%), Curvature = 100. Start Time set to 12.5% (1/2 of 25%) and End time for 50% — curvature at 100.
This is an approximation for double curvature (at least more easing at the end). The End Time of 1 and the Start Time of 2 are somewhat flexible - and should "cross over" as smoothly as possible.
You should have enough info to experiment further, if you want.