If you allow pilots with 72" and 80" aircraft to compete, it'll make mini-mac another "he who has the biggest wallet wins" affair.

I can't afford a 72" aircraft, so I'm bound to get downgraded when I fly a 40 sized plane, as it flies a lot faster, and closer, the judges DO penalise you.

I'd be more inclined to see the old F3a rules brought in for mini-mac.

10cc engine, and maximum wing area. That would be enough to keep things pretty sweet.
That way you would have such a cavernous divide between the 40 sized pilot and a 60 sized "bigger" ship. It'll also mean that a lot of the older (now defunct) pattern technology would get a look in making the planes more economical to set up.