Low VSWR at the Tx end of feed line does not indicate best antenna efficiency. Why: There would be no VSWR if the antenna accepted ALL the power, minus losses, fed to it's terminals. Efficiency it at the antenna end of the coax, you cannot affect it from the other end of the feed line. You cannot correct the antenna Impedance from the Tx end of the coax, with any amount of matching. You cannot change the element lengths, the element diameters or any other parameter from the Tx end. Obtaining a match at the Tx end is for max power transfer out of the radio into the feed line. What happens up the feed line can only be reflected back and does not change Impedance at the antenna or it's efficiency (to any significant degree). A change of Impedance does not = efficiency, they are not one and the same, not even close. VSWR does not = efficiency and not even close.