Anyone had fitter doing bad job between partition boundaries?

First thing you should try is turning off all LL regions. See what the Fitter gives you with just design partitions. Adding LL regions limits what placement the Fitter can perform. And you may not need those buffers.

If you must do this floorplanning, put the buffers in their own separate level of the hierarchy and set the LL regions to be reserved. That way, the Fitter won't put any outside logic in the regions so it will get placed in the middle between the regions.

But try your design without floorplanning and see what you get.