Massive difference once it's bedded in, I was quite disappointed with my 170 CR until it was remapped.
The ECU has to be removed though to be remapped. I was told the plus side to this was that the "Flash Count" on the ecu won't be altered because the obd port wasn't used. However if your ecu is physically inspected it will be clear it has been tampered with.
All modifications to a car invalidate the warranty of all components connected to the modification, you just take your chances.