Ok, at Knockhill, the car didn't feel remapped.
It is a little quicker than standard, but a remap would help. My Squdra'd 1.8 155 now seems quicker than standard 2l 156s.
I don't really know if anyone has remapped for high octane other than Squadra. Best thing is to ask them. However, I think that ideally there is more to it than just advancing the ignition 2-3 degrees as the different fuel will have slightly different burn characteristics with regard to flame propagation and temperature. Therefore, I think that fuel quantity should also be slightly different with high octane.
I could be talking a load of rubbish, but I think I vaguely remember reading that type of thing in a book somewhere.
Simply advancing the ignition would help, but a full remap would be better. As the chip in your 156 is EPROM, it requires a new download for each change ( a pain). Earlier ECUs could be fiddled with live, so that they could be tweaked real time on the rolling road or simply the road.
I think it may be best to find a reputable Alfa tuner, and ask if thy have done a bespoke remap on a similarly modified car. Thereby, you may still be able to obtain pretty much a remap specific to your engine 'off the shelf', and hopefully at a lower cost.