More than the HP or the money, the spindle (and cheap chinese spindles are included) will be SO MUCH QUIETER!
Like barely hear it when not cutting quieter. It's harder to install compared to a new router. You need an inverter (and ideally 220v power), a new mounting plate, and need to run the wires but you gain quiet and completely variable speeds.
RATTMMOTOR CNC 2.2KW Air Cooled Spindle, is one I have used, and the square type they sell as well. Other than they often don't have a ground wire hooked up, they are OK.
Then figure out the mounting (Sabre looks OK for this), and visit CNCZONE for setting up the VFD and you'll be good to go.
But, you can also use something like a superPID (google it, if it's still made) to vary the single speed router if you want.