Um. Which 20g is that? td05 or td06? Either way, you have plenty of turbo for that power level. No reason to upgrade. Are you sure you dont have any exhaust/boost leaks and all your gaskets are good?

How much boost do you run right now? And when does it spool? I have a td06 20g and it hits 15psi by 3100rshared_pm. Yeah I have a 2.4...but still you should be getting full boost by like 3600-3800...that is not a laggy turbo.