IIRC the 10 degree figure is for 92 octane (RON). If you are using a higher octane then you could try a couple of degrees of extra advance. If you get any signs of pinking then put it back to stock.
The reason behind advance is to set up the burn for when the piston reaches TDC, then the expanding gases give the maximum downward force. Too much and you will lose power because the burn will give resistance to the piston on its up stroke. I would say 20 degrees would be too much advance, I'm surprised that you aren't getting any pinking.
Tweaking the timing is something that people seem to be reluctant to do these days. Too much can cause damage so you have to be sensible about it, it can also affect emissions so you may need to reset it for inspection. My old Ford has an adjuster wheel on the distributor for easily adjusting the timing although back in the Sixties it was mainly to compensate for variations in fuel quality.
