The 4.3 is avaliable so if you use OTA or SEUS you get it.
As for rooting, it doesnt avoid your warranty unless you remove the root and install a clean version when you deliver the device for repair or replace it.
Only thing that avoid the warranty is unlocking the bootloader.
Keep your .257 rooted version and wait for a rooted locked version for 4.3.
Btw there will come a hotfix version in January (newer 4.3) for Z1.
4.4 will come April/May i think...
My Devices: SE W960i, SE W980, Xperia X10, Xperia Arc, Xperia V, Xperia Z1, Xperia Tablet Z