First I would try retarding your cam intake timing another 2 degrees for a total of -4 degrees. This has seemed to help a lot of people in reducing knock in the mid-range area.
Also what are your coolant temperatures looking like? Switching from the stock 87C thermostat to 70C thermostat has helped me a lot with keeping my engine running cooler and with knock.
Really though, it sounds like the exhaust side of your turbo is maxed out. I ran a Garret T04E .50 trim with a .63AR exhaust housing. That turbo is very similar in size to yours. After about 20 psi I had the same exact issue you are having. I was getting horrible amounts of knock around 4800-5000 (AFR's were in tact) and I believe this is because my Exhaust back pressure was greater than my intake back pressure (boost). I was barely able to run any timing 2-6 degrees BTDC.
I highly recommended tapping and drilling your exhaust manifold for a exhaust gas temperature sensor as well as an exhaust back pressure sensor. You should be able to with the turbo still attached, I think there will be enough room to get a drill under there and tap. These two sensors will be key in helping you determine if your detonation is from a maxed out hotside.
If high EBP is the case, you will either need to upgrade your exhaust housing (the next step would be getting a 0.72ar cosworth internal housing or a 0.82ar internal 5 bolt housing). Other than that, I would turn the boost down until you can run 8-9 degrees in that area where you are experiencing the knock. I have personally noticed, and i am sure others have that higher ignition timing and lower boost will not only be more safe but also make more power than higher boost and very low timing.
The last option is to use a higher octane fuel that will be able to handle the higher EBP or another solution such as water/methanol injection (but IMO this is a band aid for a mismatched setup).