5 psi is low, you should get 7 to 9. I would still try disconnecting and plugging the vac lines from the boost control actuator to see what boost you get. My bet is you will pick up 2 to 3 psi. Again, if you do this, make sure it is only for a short run and you reconnect the lines right away.
The tech guys on here recommend not going smaller than 3.5". Do a search on "pully size" and you will see a lot of information on this.