I am about to start this exact conversion. My Mk2S has already been rebuilt with the Haribo mod so the frame is all 3030 extrusion and is squarer and more rigid then even the new Mk3 frame, but I still wanted the Einsy board for the Trinamic drivers and the quietness they bring.
So I now have the Mk2.5 conversion kit delivered a few days ago and while waiting I purchased an Einsy RAMBo and a 24v PSU from Prusa. I did want to do a full Mk3 conversion but late on realised that not only is the heatbed different on the Mk3 (24v) but the mounting points and the aluminium Y Carriage frame are different as well.
The RAMbo line of boards including the Einsy are stated to have separate power rails for the electronics and the heatbed so I am going to build a dual power hybrid Mk2.75 thing. I am going to utilise both 12v and 24v PSU's and give Einsy 24v to the board electronics power rail, and 12v to the bed power rail. I can then use the Mk2.5 12v heatbed on my existing Mk2S Y Carriage, and have the rest of the printer including hotend on 24v.
One possible side effect of this is my dual power setup should considerably ease the loading on both PSU's.
If anyone is interested I'll post updates on progress. Wish me luck