DB Electrical has been around for a while, so they seem pretty reputable. I don't know this as fact, but I get the gut feeling that they simply re-manufacture and rebuild those alts with "hot" parts (hence the overdrive pulley).
Don't get me wrong- I did the exact same thing you did: I rebuilt my small case alternator with hot parts from Spring Hill Auto Electric too. Put a small pulley on it and let her rip. I have low voltage issues at Idle too- not really low, but none of this 14.4+ at idle. At 1,000+ rpm though, it holds pretty well. I don't compete with this thing and wouldn't be able to. Plus- I really don't trust it enough to be a daily driver.
Neither of these are close to a proper DC Power, MechMan, or etc... but- they are half the price. So... you get what you pay for.
***NINJA'd!!!***