Low Compression/High Boost and Pump Gas

Can someone shed light on the ability to run pump gas on a lower compression/high boost engine? I am trying to get my head around boost adding air to the cylinder but not adding to the overall compression ratio. Please be kind. Just trying to understand as my thoughts were adding, say 15psi to an 8:1 motor would make the need to use race gas, E85 etc. Where is the cut off? 10:1 motor? I would assume efficiency of the cylinder head and its material plays factor. And lets say Carbureted, no EFI or DFI