Remember that you can overclock the Z1 GPU to 533 MHz, so that is a <10% difference. Overclocking both CPU and GPU, the Z1's 800 (2.4GHz/533MHz) is very comparable with the Z2's 801AB (2.3GHz/578MHz) and within striking distance from the S5's 801AC (2.5GHz/578MHz). No idea on how those overclock, though.
I would have thought that was obvious. Unlocking the bootloader on my old Motorolas or HTCs provided more function and in the case of the HTCs made things better with custom Hboots. But the current crop of Sony's use the bravia engine to enhance the screen which is an integral part of the user experience and unlocking the bootloader disables it. Yes, by backing up the TA partition it is possible to revert back if the bootloader has been unlocked. But, unlike some other phones, with Sony phones the users can only choose to either unlock the bootloader to flash non-stock ROMs and custom kernels and lose the Bravia engine or leave the bootloader locked with working bravia engine but never gain the ability to flash AOSP ROMs. You can't have both because of that pesky TA partition. Because the Sony stock ROM experience is so good and thanks to the fantastic work some devs have done on modules for the Xposed framework, some designed especially for Xperia phones, Sony users have 2 strong choices.