I would have to argue (my 2¢ worth) that it is better for most users *not* to have to deal with bitrates on export. Most people get it wrong!
Final Cut does an outstanding job of providing the best imaging with the Master File export at the most optimal* bitrate to achieve that. If you want a lower bitrate, you can go for second best: "Publishing" for Apple Devices, Computer, and the lowest quality at Web Hosting (which is, in general, *not* recommended unless you can afford the "bandwidth" — better to use a service like YouTube or Vimeo).
If you *think* you know better than Final Cut, there is Compressor in which you can royally screw up your exports any way you want! There are too many options available, too many mistakes that can be made. Go nuts! (It drove me nuts...)
I used to try to use Compressor in Final Cut Studio. I learned my lessons! When FCPX came out — Compressor is the one app in the "trilogy" I did not purchase. I haven't felt any need for it (and I'm glad FCP keeps me from fiddling around with bitrates.)
YouTube has (or used to have) a *stated* limit of 8Mbps for (1080) uploads before they would re-encode the upload. It has become a lot more tolerant and I find that in the last several years, that a Master File (Best Quality 2-pass H.264) export of video from FCP does not "invoke" their re-encode algorithm (as long as the **length** of the movie is within your account's allowable "measure"). Movie files are almost immediately available within 3 minutes of upload (the overall process takes a little long as different sized version are "replicated" for their player). [If it takes longer than it took to encode your video from FCP, you know it's being re-encoded, and it won't look the same or even as good. I would not ever recommend uploading a ProRes version.]
Your example of 3.333Mbps Faster probably looks rather terrible, I would think. Considerably more blocky artifacts than you might expect especially in the "smokey" regions of the firework explosions.
*Bitrates for codecs such as H.264 are variable depending on the amount of "movement" within scenes that occur. Scenes with relatively fixed backgrounds (and lighting) will have much lower bitrates than outdoor scenes with a lot of movment, fast cars, blowing trees, changes of scenery, etc., etc. It all depends on how many times bits need to be changed to make the motion appear normal from frame to frame.
Sorry to disagree... but I personally would not like to see that feature added to FCP.
