Coffee really needs to be ground within a few minutes of use. Otherwise you can kiss crema goodbye...
The crema isn't particularly spectacular on my presso (well, it can be but then the flavour suffers) but the flavour is very close (to my palate) to what I get in a cafe using their coffee.
The water cools in a presso quite fast, it acts like a large heat sink. I would not let the water sit at all, I pour from a rolling boil into a pre-heated presso.
See below: my pre-heat routine takes several minutes - I scoop boiling water from the saucepan into the presso and it trickles back down (I prep the basket during this time).
In my opinion, for this method silicone tubing is necessary for food/heat safety.
In a test of concept I used food grade pvc but it turned opaque and warped...
This is a lot less messy than what I used to do - juggling water between two jugs and a stovetop kettle. Prior to that I plugged the spout and filled it with boiling water, but that cooled too quickly.
When the presso is too hot to comfortably touch it's about ready to my taste for darker roasts, otherwise the shots can be sour. For lighter roasts I don't heat it up as much.
I then place the prepared basket into the portafilter and use fresh boiled water for the shot.
I also pour in much more water than up to the double line - I lift the arms slightly, start pouring until it's about 3/4 full, raise completely and then press.