It might be helpful if you could give us a bit more information on the engine.
How much boost are you running, confirm which turbo, and intercooler your using, and where your injection point is?
The suggestion I was going to throw out was that you may be running too much boost, and that will not let you get to the sweet spot. Try backing off the boost a couple psi, that should move your knock point and that may allow you to get your ignition timing in a good place.
If you have logging ability it might be useful to some of the folks if you could post your logs. Maybe someone will notice something that will help figure it out.
You may also be running too much boost too early. Most knock prone rpm's should be the mid range rpms. You may also want to try turning on the spray just a bit earlier. If your turbo comes into boost very quickly you may get to high manifold pressures before the spray can really get primed and at full flow. Better to start the spray a bit early and then move up the trigger point slowly. 8 - 10 psi seems to be a common starting point for a lot of engines.
Personally 1500 max EGT is not a big deal in my book, NA engines tuned for max power will frequently run 1460 or so. I'd be a lot more concerned about getting rid of the knock than I would lowering an already moderate peak EGT number. Brief trips up to 1600 -1650 should not be a problem so a peak of 1500 is pretty conservative. Even in one of Subaru's tech pages they mention that the valves are engineered to operate in the neighborhood of 1600 degrees.
http://www.subaru-global.com/about/parts/11.html
Just shooting in the dark here, but hope that helps?
Larry