Unlike what compression does in smoothing out dynamics, high ratios in a noise will cause expansion and seem to add dynamics to a sound. How much and how these dynamics affect the sound are again dependent on the Attack and Release settings. How fast or slow the dynamics come in to play and how long or short they will last due to the release. Once you start getting on the hiss patrol, it never ends. Keyboards that are digital in nature, upon closer examination will have an alarming amount of hiss.
Some keyboards even have noise gates built in to the output stages.
The only problem here is that again they are set for slowwer releases and breathing can again be a problem. So even though noise gates may be built in , you still need an additional noise gate to remove the breathing effect. Some times the envelope of even the fastest gate may be too slow. By adding a second gate, you can tailor the envelope even closer to that of the orignal sound. Using compression after a noise gate is another interesting effect.