The only reason I can think of using the more powerful amplifier for the tweeter, is slew rate, how quickly the amplifier can change voltage in volts per micro-second. This would directly affect the gain bandwidth product, so in theory a power amplifier used at lower power will have an extended high frequency response.

In reality, most solid state amplifiers should have a 50kHz response at full power, so the above may be moot.

ATB,

Steve