Thing is, they aren't really. In my opinion the idea that there's some kind of 'custom tune' is a fallacy for a stage 1 (which is what we're talking about). You can't do 'live rolling road remaps' with the majority of cars. You just do a before rolling road, flash the ECU, then another rolling road run to check for problems and to have proof of all the extra power. It also helps to check the torque and power curves look as they should (no weird dips or flat spots). Plus you get to show this 'after' graph to your mates.
If it's a car that's a few years old, the maps have already been worked out during development. People like GAD, Big Fish Tuning and MSL simply have a lot more experience than a generic mappers that have done a couple of days R&D tested their map on a Jeep OM642 engine (e.g. Celtic) and called it a day, and are now selling them to any 'remap company' that has an ECU flashing device. GAD/MSL maps are clearly 'better' than these generic maps that just adjust a few tables and ups the boost and fuel pressure, and that's what you're paying for.
For example, on my E350, you have to physically remove the ECU from behind the wheel arch and attach a computer to the pins, flash the ECU, put it back in the car, and then test. You used to have to actually heat up the ECU box and get the glue to melt and physically adjust the soldering and so on before it was possible to do it via the pin sockets. How many times do you think they're going to do this to 'customise the remap to your car'?
What you can do is have a 'before' rolling road, chuck a highly-tested and tweaked map on there and see how it performs. If there's no problem and the gain is good, that's it, it's done. I've read this from GAD themselves over on the 'other' MB forum.
Even with cars that can be remapped via OBD, they're not 'live tweaking' anything, but it's a lot easier than physically removing the ECU. If there's a problem or the customer has some kind of 'weird' request (perhaps they really want to push the power or wants the torque or power curve tweaked in a specific way), they might get the maps adjusted to cater for that, but most people are happy with the tried and tested 'standard' map.
So, the idea that every 200bhp E250 is getting some kind of bespoke map is not really true - if you have a E63 with a different turbo or intercooler, and uprated air filters and exhaust then sure - but even then, if it's a standard stage 2 or 3 kit, usually the remapper has already worked out the best map for that combination of mods, unless you're the first!
I'm no expert though and happy to hear info to suggest otherwise.
