Originally Posted by RCZ

He has a base map/tune that works on everyone's car then he makes slight adjustments based on whatever he sees on the logs that you send him. Its more or less the same exact thing as tuning, except it takes a whole lot longer. The answer to your question is "not that much better" although it would be quicker. As an example, say he saw your AFR go too rich from 3000 to 5000rpm, he would go back to your map and take out a little fuel in that rpm range. He would look at the rest of your logs and make whatever tweaks he deems necessary. You get the new map, log a few more runs and he can see which, if any, changes need to be made. Repeat the process until he is happy with the logs and you're happy with the way it drives. Its not as complicated as people make it seem.

Some people actually choose to do it even after they have dyno tuned the car. Dynos can't accurately reproduce real world conditions, so its always a good idea to do some runs out on the road with a logger so you can send the data back to the tuner to verify that everything is working as it did on the dyno.