A greater string angle doesn't necessarily increase sustain. What is increased is downward pressure on the nut (or the bridge if that is where you've increased the angle), and it also increases string tension a bit. A steep angle makes things a bit brighter also.
Increased string angle at the nut has it's most effect on the OPEN NOTES, and this is because the open note is the one that has - as it's endpoints - the bridge saddle, and the nut. As soon as you fret a note, the endpoints are the bridge saddle and the fret you are fretting on!
If you want to affect string tone, you should notice that changing the string angle at the bridge has a greater effect. However, increasing string angle at the bridge saddle does NOT automatically increase sustain. You'll notice more effect on tone, with the steeper angle making the tone brighter. Sometimes, making things brighter actually decreases sustain.
Try this on guitars that have a Tune-o-matic bridge and a stop bar tailpiece behind it - raise the stop bar tailpiece up quite a bit so that the strings angle over the Tune-o-matic is lessened, and listen to the tone. Next, lower the stop bar tailpiece down quite low, so that the strings angle over the Tune-o-matic saddle is increased, and listen to how the strings sound then. Gibson put their stopbar behind the Tune-o-matic for a purpose - it is adjustable, which allows the player to adjust it to their preferences.
I've seen some guitarists put the stop bar tailpiece all the way down because they had READ that would give the best sustain, and it was so low that the strings would come off the saddle at such a steep angle that the string also rested on the back edge of the bridge! This is not good for tone, sustain, and especially not good for the bridge! I used to see Gibson SG's that had a Tune-o-matic bridge that had mounting studs that had worked loose over years of this lousy set up.
Cranking the stop bar tailpiece down as far as it will go for the sake of sustain is an example of a myth being created by people hearing or reading something and assuming it to be true without ever actually experimenting themselves to find out.