Seems like I literally picked the worst possible supplier. It seems as if it actually takes effort to run into a company in China who knows this little about making an MCPCB (I think I've heard okay review of Sitopway for their normal PCBs). Again, they were clearly doing lamination, but their laminate is almost certainly just thin, plain, non-thermally-enhanced FR-4/CEM/whatever. And they managed to use three different types of boards (I can tell by the resistance of the copper clad), none of them 1oz, for the order (I had 10 panels). To be fair, I still have to measure the thermal conductivity of the material, but the copper weight is definitely completely out of whack.
I came up with a list of 10-20 MCPCB suppliers (in China), and everyone who's responded is charging less or the same amount for the board, and most of them have datasheets with specs on the material... The other supplier I considered, Gold Phoenix, was charging some insane amount (more than any supplier that I've asked afterwards) for the same board. This is partly why I thought Sitopway was so cheap when I first looked. Gold phoenix had sent specs, by the way--but they were pretty lackluster.
As a side note, I've always had excellent experiences with Gold Pheonix when it comes to their standard PCBs. They're more expensive, but they do fast turn and the quality of their boards (silkscreen and especially layer-to-layer alignment) has been excellent.
Just went out and tried one of the suppliers that seemed capable and cheap. *crosses fingers*