Nice experiment, it would be interesting to keep adding higher values and see how good you can get the PF up to

Oddly enough, the higher you creep the PF up, the more you start to loose the compromise?, nudging it up to 16 mF altered the readings to 0.71PF, 47.7A, 240.4V and a circuit draw of dead on 80 watt!, you must have to compromise it dead on between too high or too low? , 10 mF seems the best?

Oh that's interesting! Maybe that is just the sweet spot where you can get the lowest current draw then? I was always told that the closer you get to unity PF (so I guess in the region of 0.98, 0.99) the better, but it is not done for the cost and size of capacitor reasons
How long had you been running it for? I recently tested 70W SON fitting (brand new, with factory fitted 10uF cap) and it got to the high 80's on my meter but leaving it longer it gradually climbed to 96 watts (which did seem quite high tbh) running a 70W SON-T HO.

It’s been on for near on 4 hours at the moment.

That should have well stabilised by then, I ran mine for half an hour and it seem to have settled on a wattage by then It would also be interesting to see what the PF and gear losses are on an SON electronic ballast

My wattmeter from Aliexpress can disply both amperage and power factor at the same time. Here is the power factor and amperage of my Semicom/Hyundai Eco T2 15W 827 colour helical CFL: