I have found the norvel 074 to be able to fly aircraft significantly heavier than 20 oz, and do so with authority. I guess it all depends what you want to do with your airplanes. I consider 24 oz to be a fantastic weight for the 074.
I'm not one to hover or break speed records, but I feel my planes are very "Airworthy"
At the risk of sounding like the former Pres Clinton, "That depents on what your definition of the word 'airworthy' is" (Done in my best Arkasas drawl )
I built and flew the Herr Mustang on an 074 with full sized gear. She flew 300 plus flights at 32 oz with almost trainer like qualities.
A HOB p-47 on the 074 was spirited at 24 oz. I never felt she was hurting for speed.
The airtronics Warlock is very fast!, and highly powered at 20.5 oz on the same engine
Digger