Originally Posted by the suicidal eggroll
Some tuners just set the car to target boost and get the car running correctly there, while the other parts of the table haven't been touched. Lowering the boost could throw you into an untuned region of your map, if so then anything could happen...too much timing, too little timing, too much fuel, too little fuel, etc.
If the car was tuned properly for all conditions up to and including the final target boost, then you'd be fine, but there's no guarantee that it was.
You guys are correct. Hopefully the tuner did tune the timing map but you wouldn't know until you looked at the map.