I disagree, it strongly depends on wind speed. I do not have any experience with the M18 but, I have watched A-Class against Cat 1 18 sq for 3 years and have sailed in it for 1 year.

Upwind:

The A-class has an advantage in very light wind


The A-class has an advantage from the point the A's fly a hull until the 18 sq starts to trap. From that point until the A's can't control it's even. After that the 18 sq's have the advantage.

Downwind.

It's even until the A's go wild. After that the A's are faster until it's too scary to wild thing (I have never seen an 18 sq go wild.)


This has been an gradual development. When the A's first appeared in my area 4-5 years ago, the 18 sq was faster. But, the A-class skippers have been getting faster mostly due to improved techniques

All of this depends on the skipper having enough experience to sail the optimum course for wind and wave.