You can find literally hundreds of posts on this subject in the archives. Search for BX with any of the following terms in this forum: rotary cutter, rough cutter, brush hog. That would give you three searches to do. I have a Gearmore 40" heavy duty rotary cutter that I use with my BX2200. It will cut sapplings almost 3" in dia. I believe it is the only heavy duty RC in the 36" to 48" size. The others are light or medium duty, yet I have heard of many of them being able to cut sapplings. If you only need to cut tall thick grass and weeds, then a medium duty 48" unit absolutely and without question will do fine.
The BX can lift far more weight than is listed for a rotary cutter in the manual. A brush hog extends out a great distance and can generate great leverage if you drive fast over uneven terrain. These G-forces can damage the 3ph and exert strain on the hydraulics if someone drives this way and abuses the tractor. No max. weight is listed for things that are used in stationary position, such as PHD, generator, log splitter, etc., because you do not use them while moving. You could lift a 600 lb. rotary cutter with no problem, but as that approaches the max. liftable weight, if one were to drive fast, perpendicularly over furrows, he could damage the tractor due to the tremendous strain created by the bouncing, especially if he hits the right resonant frequency, aka. harmonic. Kubota rightly does not want to be liable for warranty repairs if the tractor is so abused, and therefore the weight limits listed protect them. You can easily exceed these weight listings as long as you do not abuse the tractor in the manner I have described.