If UD's "inside scoop" is true, then I'd agree that it's bordering on false advertising to claim you have a 300 watt RMS power amp when the speaker impedance will never allow the amp to deliver that level of power. Most non-integrated amps have their electrical output ratings tied to the load (speaker) impedance, which is the correct way to spec. an amp. If Mackie has done otherwise, then shame on them.

While I have personally not owned the Mackie 450's I have heard about the overheating problem and that it does indeed create a limitation under certain operating situaitons. Engineering always involves compromises and trade-offs and good engineering can minimize the "bad" and maximize the "good."

Re the heat dissipation problem, some of the manufacturers of powered speakers are using fans to help with cooling and others are using the air movement created by the low frequency speaker to aid in cooling.

Cooling is certainly an issue at high power levels, especially given the compact nature of the newer powered speaker ssemblies. But so is sound quality and maybe, as is the case with keyboards, (there is no perfect keyboard, as echoed many times on this Forum) so too there may be no perfect powered speaker.
Paul C.