Eh? The only time you can max out an airframe is when some part can no longer handle the aerodynamic loads and it breaks. Drag is proportional to speed squared. If you want an airframe to go faster you need more power. You can theoretically make a Cub fly 250 mph with enough thrust, though of course you run into practical issues. There is no limit to the theoretical speed if you are saying that in some way. But yeah baby, there is a limit to the practical speed when you factor in structural limits and limits to the powerplants available.

The faster you want to fly the more horsepower you need, period. Roughly speaking dynamic thrust is proportional (didn't say equal) to horsepower times efficiency. Efficiency of course bundles some complex "figgerin'" into one number, which involves propellors of course and whatever else. So in theory a 2C .50 runnning at 18k and a 4C 1.10 running at 10K but putting out identical horsepower (horsepower not torque..) and with drag and weight being constant (so induced drag is the same) should be able to drag the airframe along at about the same speed, if you can get the appropriate propellors for each setup to work as efficiently as the other.

Mike D.