Boost adjusting?

[Mk2-matt]

Hi all, iv done alot of research and found mixed answers, iv owned, worked on and evan built alot of old turbos and generally adjusting boost on them was no issue and caused no problems (unless you went too far) but iv never really played about with or worked on a vag 20v, id like to get more boost out of it but from what i read they are very temperamental to get boost out of without spending money, mine used to belong to a lad at psi tuning (now works for awesome gti) so im not entirely sure whats bin done to it if anything? The boost gauge shows 13psi and pulls fairly well, yet looking under the the lid the only obvious thing i can see thats bin changed is the fuel pressure reg which is a 4.5bar ?