Okay, I guess my math was backwards then. Sorry about that.
Just FYI you can run 300 watts of clean power through a 200 watt cab all day long and it probably won't hurt it. It's when you run 200 watts from a 200 watt amp through a 600 watt cab that you have problems. As mentioned before, RMS handling specs from one manufacturer to another don't mean a heck of a lot.
This is really off. 300 watts continuous will overheat the voice coil whether it comes from a 200 watt amplifier (severely overdriven), a 300 watt amplifier, or a 600+ watt amplifier. This is very different from using a 600 watt amplifier with a 300 watt cabinet, since a 600 watt amplifier doesn't supply 600 watts all day long while you play bass.
Please explain to me how 300 watts will not damage a 200 watt cabinet but 200 watts will damage a 600 watt cabinet using sound physical/thermodynamic principles.
I do agree completely with your last sentence.