I've always wanted to build a 511 pump gas motor, but never got around to it. To me if they kits all cost the same, why not go bigger. There's no replacement for displacement.
I tend to agree, to a point. When you start getting to the point that the piston has to be really short, it's time to back up one. There needs to be some support on the piston to help stop it from doing "funny stuff" in the bore.