It's the torque (twist power) that mixes, not just the amount of electricity it consumes (wattage). The torque will get you through dry lining compound or hardwall on a slow speed setting, if you are sensible and do a bag at a time, but for finish you need a good whisk speed (>450rpm).
That's why 2 speed mixers are popular - but be warned, if you overload it and the whisk is not turning fast enough it will not draw enough air over the motor and BANG! You need a new armature.
The wattage is a good guide, but manufacturers do sometimes 'amend' their figures to win bragging rights. Look at the same machine in 110v and 230v - the 230v should always have more power than 110 because that's physics (110v draws a greater amperage to achieve the same work through having larger size copper wire inside - but the machine body is the same so fewer windings means less power). So if you are looking at a machine with, say, 1300w rating chances are the 110v is 10-15% below. But you'll never know...
As for speed - Perles used to sell a mixer through some outlets with a gearbox from a hole borer - maximum rpm about 350. Takes ages to knock up.
This is why some people charge you more for mixers - because sometimes we've had to do a lot of work to get the right machine. And paddle. And arrange servicing. And stock spare parts. And advise punters over the phone how to repair them. And arrange collection and return of repaired machines under warranty. Just like the man on Ebay does...
