I've also seen the same subwoofer quoted at 120 watts.
Most people think of watts as being a measure of 'how loud it'll go', which is *generally* wrong. What you want to look for there is the Max. SPL output. The BeoLab 2 is quoted at 109 dB (if memory serves) at 1m, and the Genelec is 108 dB, so the difference in total volume output is minimal.
Bear in mind though that the BeoLab 2 is servo controlled, meaning the amplifier compensates for any 'wrong' movements by the speaker cone which takes up quite a lot of power.
RE the actual wattage output, the 850 watts is not an 'overquote' (like when you see computer speakers rated in the thousands of watts), it really is around that figure. The ICEPower amplifiers are capable of huge outputs over prolonged periods despite their tiny size. The BeoLab 2 will take up quite a lot of power though, as it's much smaller than other subwoofers with the same total volume output down to the same frequencies.