There are a few places that have experience with re-tuning these.  Basically the ROM is replaced with a socket so you can swap the ROM's. There are a number of ROM editors for the RB series of engines that let you play with the mapping.  Real-time editors on the dyno are the better way.  AFAIK' date=' the RB25 computers have one-time programmable processors...[/quote'] 
  
So, you could reprogram a rb20 ecu to run on an rb25?