^ Agree. Chances are that uninstalling the HT will not change things, but it will eliminate one variable. Then you can install it back. OP is running on wastegate spring pressure. That's all he'll see if there is a big leak or if there is a leak, even a relatively small one, in the hoses and lines between the actuator and the boost control solenoid and from the solenoid back to the TIP.
You need a good boost tester, if you don't have one. It is a silicone tube the size of your TIP inlet with a plastic cap on the end and a manual boost gauge screwed into it. It will have a fitting for either a bicycle pump or for an air compressor. You connect it to a bicycle pump or air compressor and pressurize the system up to about 18-20 psi and listen for leaks and feel for them with your hands. Check every boost tube, every vacuum line, even the seams and welds on the intercooler. Check the bpv. It could be anywhere and there may be several small ones.
I had your problem, OP. I thought the WG actuator was holding the wastegate open or that the wastegate would not close all the way. I removed the actuator and tested it under pressure. It worked fine and opened at the right pressure and moved the correct distance. With the actuator off, the wastegate moved freely and was able to completely close. It only takes about a half inch of movement by the actuator for the wastegate to go from closed to fully open (which is only about 45 degrees, anyway). So, that was not the problem.
With the tester I found and fixed several small leaks and one big one. That fixed the problem. The major one was on a tap off of my stock bpv vacuum line going to a manual boost gauge. But there was also a small split in one of the hoses connecting to the electronic boost control solenoid, and also a loose clamp on the turbo exit to the boost tube going to the hot side of the TMIC.
If you really want to know, there is an excellent and easy way to check the turbo, itself, but you have to be careful. Take some ordinary bailing wire and wire the wastegate completely closed. Tie to the far end of the actuator rod, wrapping around the wastegate pin. Pull hard back toward the actuator and then tie the wire off to something rigid on the firewall above the actuator. Then CAREFULLY take the car for a drive. You must have a boost gauge to do this. You should see rapid and extreme movement of the gauge with light pedal pressure. Be careful not to exceed 18 psi or so. Get too aggressive with the go pedal doing this test and your engine is toast, though. That will tell you if the turbo is o.k. and since you know your actuator is working fine, the problem just about has to be in hoses and vacuum lines or maybe in your stock EBCS. If the EBCS does not or cannot bleed excessive boost back to your TIP, then that boost is going to push the actuator rod, open the wastegate and you run on spring pressure.
As these cars get older, vacuum lines harden and develop small splits. Clamps loosen. It's hard to track them down.
EDIT: I see you have replaced the EBCS. Make sure you ran your lines correctly. Hooking them up backward will cause you to run on spring pressure. Also, I think the Forge actuator may have an adjustable rod. You need to make sure that rod is adjusted so that it is exactly the same as the stock length measured from mounting flange out to the eye in the end of the rod. I still think you have a split hose or loose clamp somewhere.
