Allegro
More important than the speed of the sixteenths is their evenness. You can fool the listener into believing what they are hearing is faster than reality. Even Chopin himself talks about this. But this requires absolute evenness of rhythm. The piece will catch on fire if you remain steady.
Also, having a clear direction per each four bar idea will make the listener feel as though they are being pulled forward. Each phrase and rise and fall of scale passages should have its own momentum and personality.
Give the listener a great deal of information to digest as you are playing. Interesting playing doesn't necessarily equal fast playing. I would rather hear an physically relaxed, interesting interpretation with fantastic rhythm at 120 than a tight, struggling, machine-like interpretation at 144.
Also, the tempo should hover AROUND 144. You should have periods where you are a bit faster and some periods that are slower. This idea is not opposed to an even rhythm; your subtle accelerandos and ritards should be even, like an engine revving. The engine doesn't jump from 2000 rpm to 3000. It gradually gets there, and so must your tempo.
Piano (soft)
Playing softly generally requires two ideas to be in your head simultaneously.
First, there is the basic concept that a bigger motion or impulse creates a louder sound, while a smaller, more compact motion will create a smaller sound. Different technique = different sound. Same technique = same sound. So, you must simply move less.
Secondly and similar to the first idea, move slower. Many people try to play piano, yet they still use fast attacks. I want to beat people over the head when I see them do this because it is such an easy problem to fix. Move fast = louder sound. Move slower = softer sound.
These ideas work well together and they are easy to remember. So work to make your impulses smaller and slower. This idea works on just about everything, especially chords.
Legato
One thing I wish people would learn is that many times what we do physically at the piano will be the opposite of the way it sounds. For instance, every piece of music we play should feel as though it is made up of a thousand separate little pieces, with mini split second relaxations in between each note/group/phrase/etc. However, it will sound like one continuous stream of music. Those are opposite ideas, yet the listener is unaware of what we are thinking or feeling. It cannot be a continuous stream of physical work. If it is, then we will get tight or tired.
So, with that preface I suggest that you play "up". Instead of thinking down for each sixteenth, think up for lighter playing. Legato will be an illusion you create. The motion would be as if you pushed a piece of paper across a table to your friend (without using your arm, only fingers). See how your fingers kinda flick upwards? On the piano, it will sound like mini-staccatos. This technique will create a certain type of sound. The listener will hear legato, but you are doing something completely opposite.
If you want a true physical legato, I would suggest practicing the 3,4, and 5 fingers by themselves - only the sixteenths. Do this until they are connected the way you want. Also, make sure your thumb and index finger are just floating in space. I tell my students to take their left hand and lightly grab 1 and 2 of the RH to make sure there is no tension. Make sure you are playing each note to the keybed. If it becomes too loud, refer to my earlier comments. If you can't do legato sixteenths by themselves, you can't do it with the chords added. So, be patient. Now, this type of physically connected playing will make a different kind of sound than the one listed above. It will be deeper and richer. It is up to you to decide what sound you want. Listen to recordings and consider which sound they are creating and which technique they are using to achieve it.
This is a milestone piece in one's development - you will learn immensely from studying it.
Robert Henry