Unless I am really confused about preload, you can't lower boost below your spring pressure by reducing preload. If you are removing preload then you are allowing there to be less pressure on the wastegate door. You could add so much length to the rod that the door stays wide open, and as you tighten the rod down, the door would shut more and more. If you leave the door cracked a little then you will bleed off exhaust, so the turbo would require more exhaust gases higher in the rpm band to overcome the bleeding gases to spin the turbo, so in essence, you have increased turbo lag and the boost threshold higher in the rpms. If you do that then it is possible the turbo would see less boost if the engine couldn't spin the turbo enough, but I am assuming your turbo isn't so large where such a situation is possible, nor do I believe your tuner would set the rod to have the wastegate always slightly cracked. In this scenario, the rod still won't open more until boost pressure exceeds the wastegate pressure, which is still going to happen at spring pressure or 12 psi. Some springs will crack at a lower boost level and then open at their set level.
This shows a wastegate cracking open at 10psi, but then opening fully at the spring pressure of 12psi. Some springs will not crack until boost pressure is hit. External wastegates with a EBC can allow someone to run pressure to the spring to keep it closed until the last possible second. This guarantees boost hits wastegate pressure, but often there is a boost spike because there is a delay between wastegate activation and boost control, so having a spring that cracks first can be good.
Do you have a data log to share? How is it that your tuner achieved a boost level of 10psi max, but now you are hitting 15psi? What changed? Are you hitting a boost spike or is it boost creep or is it sustained 15psi? If it is something new then the most likely problem is that your vacuum line from the turbo to the wastegate came off, has a crack, has a hole from getting burnt, etc. Make sure that hose is good.
If your problems were always there then maybe he added too much preload. If someone kept adding preload then they would compress the spring more and more and more. The spring could get so compressed it would be incompressible, or it would only allow the wastegate to crack open, so there would be boost creep on top of a higher boost level, or it would just take more boost to open the wastegate, but it would open all the way, so boost would be higher, but there would be no creep.
Boost creep is when the wastegate is too small for the turbo, so the wastegate initially opens and maintains boost, but as RPMs build, exhaust gases continue to build enough to spin the turbo more, so boost rises.
Here is boost spike.
