You have to positively engage each gear, if you let it skip over the dogs, you can damage it. Though not a scooby, I have the PPG straight cut set in my high HP DSM and drive it on the street frequently, when I'm home anyhow.
Don't let anyone tell you that you can't downshift, or if you miss a gear you have to come to a complete stop to re-engage any gear, drive it like any other transmixer, just make sure you don't limp wrist it onto gear and you'll be fine. I don't know why, but those are the two primary myths I seem to come across about dogboxes.
You have the ability to no-clutch shift with a dog box too, preload the shifter, activate some type of ignition or fuel cut to momentarily slack the drive train, and pull the next gear like you mean it, it WILL go.
The first time I tried it off the 9750RPM rev limiter 3 things happened. First, there were bad noises, like really bad. Second, the car turned 90* to the left and upon later inspection, actually laid rubber. Third, the pucker factor activated. I drove the car back to the garage, jacked it completely off the ground, and inspected the drivetrain. All was well, though I still haven't mustered up the intestinal fortitude to try it on the track... yet. It is just begging for broken axles and t-cases, I can't wait.
I have about 3500 street miles on it and haven't even found "normal" geartrain wear particles in the oil.
Straight cut such as mine are loud, the helical should be quieter. Helical gears are technically stronger depending on tooth profile since they usually have a tooth and a half in contact, but they impose side loads into the bearings and case, so you may find more wear there. Straight cut only contact 1 tooth at a time, but does not put the side loads into the case.
I would use the exact thing I have again in my specific build, and wouldn't mind DD'ing it, but the whine will eventually drive you crazy. I have 4" turbo back exhaust, and cruising around the gearbox is much much louder than the engine outside the car.
Here's a video of me out cruising around.
http://s85.photobucket.com/albums/k5...rent=Drive.flv
Again, I know this isn't Subaru specific, but I figured it could help.