I upgraded my 300 a month or so back and noticed no improvements (I hoped it might speed up boot time but hey, 140 seconds is fine isn't it!)
Personally I'd only update again if I have an issue with a particular blu ray disc.... firmware updates can go wrong and wreck the player.
My update appeared to have got stuck and didn't go according to the Sony instructions but after waiting 20 more minutes and noting that nothing was happening I unplugged and powering up, the firmware had changed and all worked. 'Phew' I thought to myself