I've learned a lot of great information about aviation on this website. One insight I learned from a poster here is that, all else equal, for every doubling of engine power that you give an airplane, speeds increase by about 25%.
However you're not always comparing "all-else-equal" airplanes, for example sometimes I want to compare between a single engine airplane a twin engined airplane. Yes the second engine adds weight and drag, but the factor that I have the most trouble accounting for are the difference in aerodynamic requirements between the airplanes.
My understanding is that regulations require all single engine airplanes to have a stall speed of no greater than 61 knots, while a twin-engine plane could have an arbitrarily higher stall speed. The higher stall speed is directly correlated with higher aerodynamic efficiency, meaning that it should be able to achieve higher speeds than a single engine airplane of the same mass, drag, and power, would be able to achieve.
So I am looking for a simple rubric with which to be able to compare single and multi engine airplanes. I think that there are really two parts here: (1) identify realistic stall speeds for the multi-engine airplane and (2) formula for comparing top speeds between planes with X mass and drag and Y power and Z stall speeds.