Hard to say exactly how much efficiency will be lost as it depends on several variables. Impedance of the coax, impedance of the balanced line, SWR on each line, RF currents in each line how bad the balance is upset etc. What I am saying is that by unbalancing the line by using common mode chokes on each end rather than baluns is that the now unbalanced ladder line will radiate as part of the antenna system and signal will be lost on TX and the feedline will pick up signals as well as the antenna.
The balanced line will have equal and opposite currents as long as it stays balanced. throw an unbalanced condition into it and those currents are no longer equal or opposite.