Not experienced in this field or anything, but to me this points to a well calibrated stock map.
As I assume you are running a slightly rich AFR and have the capacity to deliver that amount of fuel, which you should given the mods.
So as a result, the point of advancing timing in terms of power gain, i thought, was to time MBT to occur just after tdc. however if you are advancing timing and netting less power, perhaps you are advancing it too far.
Realistically the only way to know is to tune on a dyno, whilst monitoring your afr, to see where you are gaining or loosing power respective to tweaks in whichever direction.

I'd be cautious of advancing the timing without some way of detecting knock, when I had my s40 mapped the mapper put his own knock sensor on the engine which he then had headphones in whilst mapping, he could tell when it was starting to knock and would then back off the timing a little to create a safe zone.
I could have squeezed another 5bhp out of my engine but been on the limit in terms of knock so we decided to retard the timing a little for safety.
I wouldn't personally start playing with the advance without using a rolling road and the proper equipment but each to their own.

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Goldie the track car.

Tks, people. The car will get rr in a couple of weeks.

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Life begins after 200cv... Xsara VTS 2.0 16V: 202,5v