20bhp would be amazing gains, But just from timing and fuel calibration its not going to happen on a 1800cc NA engine.
Ask them for a dyno graph of a stock map and there tuned map layed back to back.
8-10bhp tops on the peak power. Plus a good NA map will give huge gains in throttle response and ans mid range power delivery.
For £200 you can get a remap and rolling road session...
Nick