I keep reading everywhere that 6dB is the target?
Sky's default is 6.9, O2 use 6 which accounts for some of the increase in speed seen when moving from Sky to O2. Your noise margin will come down as the speed increases, at Sky's 6.9 you should see a speed of 12-14Mb, depending on your line. If it is very good you could see 14-16Mb.