Ok let me take a stab at the problem.
First off the nomenclature of a "buck-boost" normally means a transformer which is used to boost V because of V sag over wire length .
What Your asking is for a buck boost converter . What that entails is quite more complicated then any ole' transformer.
The process that you need is one that includes regulation at input , so you won't over V the circuit. The second part is a boost converter that uses inductors to supply the "missing" V, the out of which is negative.
You must employ a smart Voltage sense which monitors V and tracks Input V-C
The last stage is a VR that can supply a V down to a few mV.
Finally to your A limits Is this the draw or the supply? Details are scarce here , perhaps if you explain what your attempting to do , it would be more helpful It appears Your trying to build a platypus.
The answer to Your question about efficiency is--no , any time you go through multiple stages of conversion ( especially utilizing transformers) the efficiency goes downhill.