I'm happy to report that my S2 now works as well as my old Desire :)
having updated my firmware to the KI3/4 flavour - the sound level out the headphones is up to the same level as my wife's HTC legend (10dB better than before)
The GPS (long time to lock issue) wasn't automagically fixed when I did the upgrade but for the past 2 days it is as good as my old Desire.
I don't know what I did to get it into this state (as I've fiddled with so many things) but I might try and break it to see if I can work out the exact sequence needed to get one from poor to good.
One piece of info though - when it was poor, the GPS indicator didn't come on for about 5 secs when a GPS app (e.g Maps) was launched - now it comes on straight away (like my old Desire). I'd be very interested to discuss settings with anyone who has a 5 dec delay when launching an app that uses GPS.
A happy Simon :)