The impedance of the speakers varies with frequency. Usually with reasonable quality speakers/amps etc there is some tolerance. The connection you describe is unlikely to cause problems.

These are the possibilities:
Excess power into the speakers long term could cause overheating and damage to the voice coil - like heavy rock on max for a good long time.
Excess power into the speakers short term is likely to make them rattle a bit
PDQ is partially right - if a low powered amp is turned up too high in an attempt to get enough volume, clipping will occur - this generates harmonics which increase the high frequency power through the speakers, often destroying tweeters (paradoxically bass overload causes most high frequency harmonics in a typical music programme). This is the opposite situation to yours however; I would agree with the practical solution of using amplifiers of a high power rating.
Doubling or halving the speaker impedance doesn't double or halve the power - the amplifier has current and voltage limitations (internal impedance). Sometimes a 'perfect amp is postulated whose voltage/current is not affected by load.
Often the term matching is used to describe the 'correct' load for aa amplifier - optimum is better.
It all depends as well on what ratings are used - if amp and speakers use RMS wattage ratings, you're OK - if the amp is RMS and the speaker is PMPO or someting similar, forget it

Stu