So today I have calculated my steppers for XY that I have right now:
Approximate peak back EMF due to rotation per motor: 7.8 V at 200.0 mm/s
Approximate peak back EMF due to inductance per motor: 1.9 V at 200.0 mm/s
Step pulse frequency: 18.8 kHz at 200.0 mm/s
Speed at which torque starts to drop (low slip angle): 174.2 mm/s @ 23.2 kHz
Speed at which torque starts to drop (high slip angle): 236.8 mm/s @ 31.5 kHz

This means that I don’t need to change them? And how to take into account weight of the carriage?

P.S.:
One more crazy idea was to split carriage for 2 independent extruders. But that will give me more weight on Y axis..