Boy did I lust after the AdSpeed in magazine adverts at the time. I don't think it was ever good for 2x speed, maybe closer to 30% because it had the onboard cache? That incompatibility though! 
I also had the Adspeed. I added the optional switch so you could "switch it on the fly". On some programs the difference was hardly noticeable, but I remember some of the scenery generating programs really picked up speed and some flight simulators ( I think A-10 Tank killer, Wings, and others) increased their frame rates to a usable level.
