Leads on this are pretty cold... it's a SuperMac SpeedCard
Thanks to this eBay seller we know what the driver diskette looks like, and handily the front cover of the manual shows a screenshot of the System 6 control panel, indicating that the cdev is likely just called 'SpeedCard.' Perhaps they or a future buyer could be persuaded to image the diskette.
Other than that, there was mention of one in a collection here in 2007 and another on arstechnica in 2002. It's a long shot, but if you want to track them down it might still pay off...
The better Nubus cards were sold for specific tasks and gaming was not one of them.
Has some interesting information of the Q700/Q9x0 video.
For gaming of that era moving bytes around in RAM is all that was needed (high bandwidth) and built in video was the fastest for that.