Was able to get 1506/4000 stable at 1.2 v as far as I can tell. Tested it with various 3D benchmarks then customized my rom to 'Bake-in' 1506 as the default TDP Boost clock entry. Unfortunately for some reason while gaming it Boosted down to 1341!
I couldn't understand why it would Boost down or throttle as I raised my TDP values in MBT but it was using clock state #61 instead of #74 which I had set to 1506. So I had to shift down the values in the Boost table and shift up the voltage table to match.
Now it will auto Boost to 1506 and throttle down normally but I kind of cheated here it feels like.
If anyone knows why please comment. Attached my mod rom for anyone to take a peak at..