In my experience the wastegate actuator can only be adjusted to about 6-7psi
on non intercooled cars. For intercooled cars the spring in the actuator is
much stiffer letting you adjust the boost much higher. Since you are only getting 5.5 this may be all you get with yours. Make sure you are measuring the
boost after the intercooler on the nipple on the metal pipe that goes to the
throttle body. If you are reading 5.5 psi there then that is probably it. The
intercooler itself drops .5-1 psi and that would mean you have upwards of
6-6.5 coming out of the turbo.

With nothing else wrong you can raise your boost by either changing the actuator to one with a stronger spring or make a Dawes Device/Gus Mahon boost controller. Do an internet search for 'Mean Mini' this is the turbo minivan that Gus races. Also I think there may be a site called www.Dawesdevice.com.

Personally I went around and around with this. I finally bought my own factory
intercooled car and found out for myself how much stronger the spring in the
actuator is. In my non factory intercooled 82, I raised the boost after I
installed an intercooler by building the Gus Mahon device. It works very well
and was very cheap to buy the parts for. Make sure you keep some kind of accurate boost gauge around as you adjust boost. Also I recommend installing the
overboost cutout from an intercooled car because this raises your cutout from
10psi to 15psi. Hope this helps you.
Dave 82 242ti 263k mi.