We tried the 20, and running correctly with all the stock parts and an APC 9-4, it goes about 3.8 sec/lap on 60 foot lines.
I'm going to have to try mine again now. It didn't seem that fast to me, but when I built it I was in the habit of going pretty quick.
Brett probably knows better than me, but on the bigger engines you can get the same effect of a smaller venturi by piling layers of nylon mesh onto the venturi intake. This allows you to tune the "venturi size" fairly easily at the field when you're getting a new plane dialed in.