SQ difference could have a lot to do with the phones being used and/or external amp paired with. In my case I use JH16 and T1 most of the time. Been switching between 3 amps (O2, UHA4, C5D) but the FW experience has been consistent.
1.2.2 - fairly flat
1.2.3b - muddy bass (the least FW version I like)
1.2.5 - in between the 2 FW above. Extracts more details across the FR. Just a hair more bass impact than 1.2.2 but far from the mess with the bass in 1.2.3b.
1.2.6 - similar to 1.2.5 but has elevated bass response and still nowhere near that of 1.2.3b.
From my experience the FW updates barring 1.2.3b is a slow trickle of sonic improvement.
Still using 1.2.2 and will stay with it until the real thing comes along. Thanks for the information as you hear it.