What i've noticed is that the older DJ read way high, while the newer units seems to read about right. We tuned a buddys 911 on an older DJ and switched to a newer one, and lost 80hp.
The older DD units read very low, while the newer ones seem to be a little happier, i know something changed in the way they were calculating everything because a few buddies of mine all said they started to suddenly read higher after a software update.
The mustangs seem to read on the conversative side(about right in my book), new or old. The land and Sea units(if set up properly) read about 1-2% higher than a mustang, but are very easy to munipulate to be made to read higher. The superflow and mainline units also seem to read about right. I've never used a dynocom or DP unit.
All dyno owners think theirs read right, including myself. I can calibrate mine by hanging weights on it, last time i did it, with 1000lbs hanging on it, it was off by 1.14lbs. That's less than 1%, good enough for me. A local machine shop has an engine dyno, using calculated drivetrain losses, were within a 1% or so of each other.
It is kinda funny that you menton those happy reading DP units, I hear that alot.