I have a Telstra I9100T in Melbourne city. As far as stock roms go, I've found that XWKI4 (which has the XXKI1 modem) has given me the best performance. On average I get about 6Mbps down and 2Mbps up with peaks of 8Mbps down and 3Mbps up.
The stock rom XXKI3 (with XXKI3 modem) I was getting terrible data speeds. Averaging 3Mbps down and 1Mbps up. This was very noticeable as I tether my laptop to my SGSII at client sites.
As for the difference between 2.3.4 and 2.3.5, there isn't a lot. I actually prefer 2.3.4 which doesn't have the vibrate icon on all the time and less of the blue scroll glow.
I have yet to try XWKI8 with the XXKI4 modem. Hope this helps those who are still deciding amongst the plethora of roms available.