Reprap variants with servo motors rather than stepper motors?
You can get "stepper replacement" servo drives that supposedly put all the needed control in the drive, and accept ordinary stepper inputs. Those should make the servo-drive a "drop-in" option on anything that uses stepper drives.

That said, I've seen an affordable CNC router system based on steppers turned into a much more expensive system based on servos, and I still have the "obsolete" stepper version, as the servo based system is MUCH more expensive and I could not keep up with the "upgrades" and never would have bought it at the price it now goes for. I'm in fact considering changing that to a reprap control system (still as a router, not as a printer, as I currently think.)

Given some practical limits to printhead speed in additive 3D printing based on the material solidifying, there may not be a lot of benefit in the considerable added expense of servos. They can move faster, but how much of that will translate to actually printing faster? How fast can you melt and pump plastic and have it stay where you put it?