The impedance of the speaker (in ohms) determines the load it will place on the siren output circuits. A lower impedance speaker will draw more current and therefore take more power. You can easily calculate the theoretical values of current and power using Ohms Law.
The power rating of the speaker should not be taken as an indicator of how loud it will sound; a 1W speaker might be more efficient and therefore sound louder than a 5W model of the same impedance.
If you overload the amplifier, its output devices will be stressed and possibly damaged. If you overload a speaker, it too may be destroyed or damaged. So be careful. Do some calculations!