Default Upgrade from MK5 to NF1 firmware

Currently I have installed the firmware for i317M Telcel (MK5) and want to know if it's worth upgrading to the new firmare KitKat (NF1). I have already activated Knox counter, but I read that if I update to the new firmware I can not use custom ROMs or stock that are not based on that firmware.

So worth the full firmware update or do it for custom ROMs and manually update the modem?
Device: Galaxy S I9000
ROM: Official SlimKat Weekly

Device: Galaxy Note II (i317M) LTE
ROM: KitKat DN3 V5

Device: ASUS Nexus 7 Wi-Fi (2013)
ROM: Official KitKat Rooted

Previous phones: Sony Ericsson Xperia X10 | Sony Ericsson Xperia Play | Samsung Galaxy S2 | Samsung Galaxy Nexus | Samsung Galaxy S3 | HTC Droid DNA | Samsung Galaxy S4