Indeed, reception will depend on the radio firmware more than the OS version. This will probably have received an update at the same time as the OS but may not, and conversely may be subsequently updated separately from the OS. And as
@MLSS says it will vary between device.
Signal bars however are a poor indicator of reception because there is no standard - "3 bars" can mean different signal levels (in dBm or ASU) on different devices, or even different software versions on the same device. More than one manufacturer has "improved reception" in a software update just by showing more bars at the same signal level or increasing the time over which the signal is averaged before updating the bars (the latter actually is sensible if not taken too far, the former is purely cosmetic). The real test is whether your ability to hold a call in a location you know has a very weak signal has improved or degraded.