I don't agree with the numbers for the MSX250 (subsequently the msx300 would be overrated as well). The msx250 (which is basically the msx300 with a slightly smaller body diameter) would no way be able to handle those huge numbers. I feel comfortable putting the 250/300 on a comfortably stocked 300 gallon tank at the most. It seems a perfect match up to that point. Larger tank would require another skimmer, or a low bioload, or a highly productive and significantly sized refugium.
I find the msx300 to be barely keeping up with my heavy stocked 220 that I have it hooked up to at the moment. Its pulling a significant amount of skimmate out but it has not been able to "run out" the tank of skimmable organics. I have some massive skimmers that will stop pulling skimmate after being hooked up for several weeks to a tank. It will stop being able to produce a foam head until more organic are introduced into the aquarium (be it more load, feedings, etc). The msx300 is not able to outskim my 220 so I don't see how it can go much higher. Don't get me wrong, its keeping my tank clean and the water clear but thats because its near its upper limit.