Not an M3W but I find the 99octane (usually V-Power) does seem to improve things. Plus8, Rover V8 with Gems management. I was occasionally getting a random intermittent misfire fault. Checked all ignition and as a final test changed to the 99 from 95. It worked. I don’t feel any performance/ power improvement. Not doing a high mileage means extra cost is not an issue. I put 95 octane in if no 99 is available.
I think the benefits do vary with engine type. A mate has tried it on an outboard motor and spotted less vibration when used.
As an analogy I used to commute 60 miles each way, mainly motorway. Had a Focus turbo diesel. I experimented with Millers additive ( advertised as an octane booster). It worked. Using the recommended dosage from the bottle I got enough extra mpg to save about £24 per bottle on fuel cost (a few tankfulls per bottle). Bottle cost about £12 and extra mpg gave about £36 in fuel saving. A mate tried it on his Golf but no difference.
It could be worth giving the 99 octane a try over an extended period and take notes before and after. Compare mpg, how engine runs, similar mileage/route to get a decent comparison by eliminating variables.
