There certainly used to be - not sure if you can still get them. I've got an AmD one from 1998 in mine - wow, hard to believe having written that that my car has been remapped for 10 years now!!! Anyway, that was old, old, old AmD (when they were in the village garage in Islip) From memory, I got about an extra 10bhp and 8lb/ft of torque. What improved immensely was the overall driveability and throttle response, and when driven carefully, an improvement in MPG.
OK, you don't get the big BHP increase numbers you'd get with doing it to a turbo, but still worth doing in my opinion. Mine wasn't a piggy back - rather it was a complete replacement ECU. Cost me £199 and well worth it.