Well I finally got back to providing the answer that I said I would in my earlier post. I rented a CAT 299XHP with HM315 for 7 days/40 hours. Had a couple of issues that slowed work down. Found part of a 1 inch diameter logging cable about 15 feet long, well the 315 found it, and wrapped it around the rotor. Had to cut it out in pieces; after I drove 30 miles round trip to get the equipment to cut it out. Started to shed a track, not fully but started to, thus that slowed things down while the track was put back on. The electrical connection from the 299 to the mulcher was "grabbed" by limbs and "removed" from the canon plug connector. Had to figure out the pin out to replace the wire pins.
Oh did I say it rained for all 7 days and the light level was low.
End result was, in uneven terrain with lots of CWD up to 6-8 inches diameter, I finish a little over 5 acres in 34 hours on the machine. Could have done more if I had not had the mechanical issues but stuff happens, especially in the environment I was working in. Very glad the glass was shatterproof on the cab.
I don't know about the 279. Think it is underpowered for the 315 based on my 299 experience for the week.