To adjust the TiAL BOV you have to change or shim the spring. Not very user friendly from that point of view. They only have 4 to choose from and you likely have the one closest for your application. TiAL BOV Springs

I think the other problem is the STS rear location causing the selection of the turbo to be prone to surging at low exhaust flow rates without having noticeable lag at lower rpms. My T60-1 still goes into surge if I lift abruptly even with the BOV but not as bad or for as long a time as it did without the BOV. I guess this does not become too serious an issue until you pass 12psi boost levels.