I don't think the 20mm/30mm option was a big deal last FSO, mostly due to the weak armor the zekes have anyway. I'd much rather have a p39 slightly lighter with a faster ROF and better projectile velocity.
The AH scenario flyers are among some of the most gracious and enthusiastic people on AH currently. I strongly doubt anyone would circumvent the rule set just for their own pleasure...
Do rules get broken? Yes, but often because things get so busy in a scenario event. Take DGS for example, Easyscor's bombers and some of our P38s were heading home, due west along the southern border. We fought the Luftwaffe for over 150 miles all the way to friendly airspace but in the firefight the bombers had neglected to notice the southerly winds had pushed them off course slightly and into the no fly zone.
I tend to give people the benefit of the doubt...