can anyone explain to me what the purpose of the solenoid is, and what affect it has on the boost pressure or turbo? And what does an aftermarket electronic boost control do by bypassing the solenoid?

The factory boost solenoid does two things: it monitors manifold pressure, and it vents pressure if needed.

From my understanding of the electrical schematics and vacuum routing, the manifold vacuum tap goes to both the waste gate and the boost solenoid. The PCM uses this signal to monitor manifold vacuum/pressure. The other line from the boost solenoid vents manifold pressure back into the intake track.

I'll give you my real world example of what it can do: I installed a homemade manual boost controller (ala Turbo Gus). With the factory boost solenoid plugged in, the boost would spike to about 16 lbs, then went back down to a PCM controlled 13.5 lbs. With the boost solenoid line blocked, I can now hold 16-17 lbs by using the boost controller.

Does that help?

BTW, I'm sure I'll incur the wraith of the suppliers here, but I suggest making your own boost controller rather than paying $70+ for one. For just the parts, I paid under $20 for my boost controller. Can't beat that!!!

How exactly does one manufacture a boost control...?

poot!

http://home.earthlink.net/~turbogus/index-old.html

Visit the site and it will give you directions. For the WRX, you'll have to disconnect and block the vacuum line going to the boost solenoid, and put the boost controller inline to the wastegate.

I ordered my parts from McMaster-Carr and got them the next day.

BTW, Turbo Gus was killed in a motorcycle accident last year, so make sure you give him props if you use his instructions.