At the same clock speeds, a Go 6600 ought to perform exactly the same as a 6600GT. Same number of pipelines between a Go, GT, and regular 6600, just different clock speeds. The Go would possibly use a little less power though, but that would be dependent on the card being designed properly for it to matter.
There've been a few cases over the years of "misleading" labelling and advertising where mobile chips were used instead of the desktop versions. I'm not saying Inno3D actually DID use a mobile chip, most of the time things like that are discovered and spread around the Internet pretty quickly. They may have just not programmed the BIOS ID properly or the chip may not have the proper ID so the drivers are mis-identifying the GPU; Windows may not be reading the same ID information for Device Manager.