[rotrex]

in honest opinion and experience, spraying water though the turns of a plenum leads to streams of water running along the walls with little effect on combustion., hence it wonÄt accept more timing.
Seriously, try direct port injection with the jets located in the final straight section of the runners on the top side near the fuel injectors.
For the same flow, you get way more effect and it will accept a lot more timing.
But as you can only globally add timing, you will have too much advance at part throttle.
I would also seriously consider programable ECU. There are plenty of options for VAG ECUs to be remapped.