I'm not of the experience level to comment on the pros and cons, but I can offer an interpretation of the rule as it is written.

The prop rule specifies the required strength and other characteristics of the prop.

However, the 7.4 x 8 props come under the heading of "Prior Approval".

This indicates that other props could be allowed as long as they meet the required strength and other characteristics, and approval is granted by the Pylon Racing Contest Board.

I believe the 7.4 x 8.0 was allowed and approved because it was not a rule change, per se.

So perhaps a "Rule Change" is not required to allow additional sizes. To allow modifications to the prop would be a rule change.