With regards to EMC, it has some benefit to connect the heat sink to the same ground as the mosfet drivers and in some cases the sources/emitters, this makes sure the capacitively coupled noise from the drains/collectors can return to the ground point without spreading. This is especially important with higher switching frequencies. I think the two capacitors are sharing the total voltage, so If you connect the mid-point to the output, perhaps there is increased risk that one capacitor will see the full output voltage. Looking at your diagram I notice the rectifiers in the left side will act as bridge all the time, independent of the transistors, so it will not be possible to make a software based harmonics reduction algorithm or integrated soft starter. It seems the transistors in the left side can only help assure the mid point voltage for the left side capacitors. If that is the case I think you could start experimenting with the short circuit of the midpoints of the two capacitor pairs and optimize on the associated control software.

On the spur of the moment what comes to my mind is leakage currents, due to any imbalance.