What does the boost solenoid do exactly??
 

as above, and how does it operate? Can you get uprated ones that do anything better?

Put very simply, the boost solenoid is nothing more or less than a switch that allows air bled from the turbocharger compressor outlet to reach the wastegate actuator, thereby opening the WG.

The ECU controls it, switching it on and off very quickly to vary the amount of air that reaches the actuator, thereby allowing it to regulate the amount of boost your turbo generates.

Yes you can get uprated ones, the main benefits being more accurate boost control (especially at higher than standard boost levels). Plenty already written about them so search is your friend.