On an engine like yours I think it probably would make a difference.
Putting it in an old V6 Omega wouldnt.
Agreed....Signum V6 seems to run fine on cheapest supermarket shit. Returns around 26 MPG according to the trip computer.
On the 230 mile round trip to Viezu the BMW trip computer showed 35.2 MPG......including the rolling road tests.
Back in the day my old Triumph 2500S would manage about 13 MPG around town and probably less than 20 MPG on a run even when using 'overdrive' as much as possible. 
My 3.2 Elite seemed much more eager with better throttle response when I started using E5 99 RON fuel instead of the previous E5 95 RON (before E10).
It has had nothing else but E5 99 since.
I get more mpg than that towing a 13' caravan with my current Triumph 2500S. Maybe you had a real lead foot at the time or a more youthful attitude to using its performance than you would today

(!!) or maybe it wasn't set up properly, but I'd struggle to get it down anything like those figures!