Welcome to the forum. Sorry no one has chimed in sooner but there are several things that could be affecting low turbo boost. You could have leaking CAC boots (those silicone hoses that connect all of the large turbo tubes together). You could have a worn out turbo. You could have leaking up-pipes from where the tubes coming from the exhaust manifolds connect to the rear of the turbo via the "baby's bottom" collector. You could have a low power motor that simply can't get up the power to produce any higher boost in the turbo.
And, as you mentioned, it could be that the waste gate controller is bleeding off the the turbo pressure before it needs to. I'm not familiar enough with how the Banks Bighead works but in reality unless you've got larger than stock injectors you're probably never going to make enough boost with a basically stock motor to even have to worry about needing a wastegate at all. It just ain't going to happen. If you were to do some mods to the motor enough to push yourself up closer to the 30lb area and above, I would then be more concerned with dialing in the waste gate accordingly so as to give you the power you want without damaging the turbo. But, probably by that time, you'd have a better than factory turbo on it anyway and it could handle more pressure :-)
There is lots of information on each of these areas that you should be able to read up on after doing a search or two on each. Good luck!
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Last edited by losfinch; 09-13-2018 at 10:14 PM.