I dont have a silvia, but ill have a go at answering anyway.
First off 107 deg sounds a bit high. If you could flush the group head a little longer until the temp drops to around 95 and then lock in your portafilter this may help.My question is, how do you go about changing the brew temperature to achieve the optimum taste?
A general guide to brew temp goes something like this - if the temp is too low the espresso will taste grassy and acidy and will most likely be sharp and have a rough mouthfeel. Too high and it will taste woody and possibly bitter, as well as being pretty flavourless.
So the idea is to find a balance between the two which displays the coffees inherent characteristics.
Also good to keep in mind that there is a relationship between grind - dose - and temperature. So make sure that your grind and dose are exactly the same each time your experimenting with different temps, otherwise you will drive yourself mad.