Airplanes are compromises: They’re never large enough, fast enough or light enough for all potential customers. And going faster is always a popular upgrade. After all, we don’t fly airplanes to go slow. Of the two basic ways to increase top speed—clean up the airplane’s aerodynamics or stuff in a larger engine—each has their benefits and drawbacks. But when one method has been tried, or reaches the point where further refinement isn’t cost effective, designers turn to the other.

Mooney’s M20 series is something of Exhibit A for this practice: The basic airframe had been cleaned up over the years and powered by different engines, resulting in iconic models like the…