Add more sealing goop aka high temp RTV or change to an O ring design. TZs use a pair of nylon (?) O rings and they still allow oil to seep out, Make sure the retaining springs are strong enough and seal the slip joint. A double slip joint helps but goop is usually enough,
That last 21 degree run is interesting in that it's the same advance at 7500 as Bill's curve but presumably the pipe isn't as hot as revs rise causing the motor to not clean out properly. That way it would be fat all the way because it isn't burning all the fuel. Excess fuel will burn in the pipe but will restrict power. As the timing gets closer to ideal, combustion should be more complete which should raise power and engine temperatures.
If the steel plates start to slip you could try machining the ID to make them more like the other two plates you showed.
Great progress so far. Don't stop now. There's more HP hiding in there, waiting to be unleashed.