Not sure if it's different for Nvidia cards, but not all AMD cards seem to be alike. In the case of the 6950s, there were two reference PCBs, which made it really annoying to find full-cover waterblocks. On top of that, companies like Sapphire made proprietary PCB designs that were not compatible with any other coolers or full-cover waterblocks. And those proprietary PCBs came with a locked bios for the most part.
I had an MSI 6950 with the original reference PCB and reference cooler, then later bought a Sapphire 6950 with a "good" cooler -- only because I got it cheap on ebay. I cooled both with an MCW82 waterblock and ramsinks. I was lucky that the sapphire had a bios switch and unlocked shaders, but flashing it with any other bios was not possible. The voltage was capped at 1.18, it didn't overclock well at that voltage, and GPU-Z couldn't monitor the VRM temperature.
The MSI, on the other hand, was perfect in every way. I'm betting the stuttering I got using crossfire was mostly due to that Sapphire card. It was definitely a noob-oriented card, meant to provide good cooling with an extremely modest overclock, nothing more.
This post was edited on 1/7 at 10:00 pm