[Magertron27]

If you can, verify that all boost control hoses are correctly routed and pull the terminal off the battery for 10 minutes or so and do a battery monitoring reset as-well. Some issues with these cars come around because of the battery being funny.

If you had a boost leak it would be a very distinct whistling, not that kind of fluctuation of boost pressure.

 

[Magertron27]

That's pretty normal for an aftermarket downpipe, I fixed mine by installing a defoulder on the front O2 sensor.

Elusive Racing - O2 Sensor 90 Degree Defouler Extension

Also, if you have a catless downpipe there is not anything there to get clogged.