kcook,
We bought PowerTrac's 4' "brush hog" mower for use with our PT422. We have only used it for about 4 hours, but it will indeed cut brush up to about a 2" diameter.
We've got a field of 3 or 4 acres that had been allowed to grow up for about 6-8 years (some before we bought the land, some after) and there are pine trees and others growing there now. General height for all of them is 5-6 feet, I'd guess. What type of tree the others are, I'm not sure, but I've heard them called "thistle" trees as they have about 2" long thistles all over them.
In what mowing we've done on this area, we've taken down quite a few of them (20-30 of this size, many more smaller — what I'd categorize as "shrubs") with the brush hog. Now, most of this was done in cooler weather (40-50°F) this fall/winter, so this may account for some of the cutting ability — as I would expect them to be more brittle than when the saps running. To back this up, the cut trunks appear to have some indication that it "snapped" part way — not a "smooth" cut.
Generally, it took two to three passes over the limbs to chunk them down, but the brush hog took it all in stride. No problems other than a stall once or twice as I tried to chew up too much too quickly. Blades are in fine shape afterwards.
Hope this empirical evidence is of some use.
Brent
I will also advise anyone who plans on using PowerTrac's brush hog to not have anyone nearby when mowing down any trees, though! I had pieces 3 foot long or so thrown 20-30 feet!