Engine design limits the boost pressure. Compression ratio, cooling problems. If you dropped compression and did head studs you might be able to push the boost a little bit. When that engine was designed people weren't building powerful diesels with high psi. Since you live in aus then you can do it fairly reasonable. Even 7-8 lbs of boost will take you from 60-70hp to around 100. Its not going to be fast but it will get you up the hills then.
