Actually, piggybacks are suited for cars that have no options to tune the ECU directly. As the ECUs cannot be tuned, piggybacks like Unichip/RS is used to override the ECU settings.
For reflash, they basically take some SAME standard mappings and load it into every same model, it would optimize some settings based on road conditions & car, but it's still far from being optimized. Since every car is built differently (even same model), reflash doesn't bring out the best, just some parts only. Eg: FD1 global map, vs FD1 racing map, vs FD1 Europe map etc etc.. it's still by a 'category'.
For ECU tuning, for cars which are able to be done so, is by extracting the existing ECU map, study it, and optimize the map before flashing it back. This is the optimized version, which best suits the car and the road driving conditions.
When you do all these, if you don't know what gains you are looking for, it's quite pointless to monkey see, monkey do. People often talk about CAI, decat, open pod etc... GOOD GOOD MUST DO VERY GOOD...
But mostly.. don't even know what gains they are talking about. Just blind following won't help.
If you don't know what you're doing, best is stay stock. I'm not a guru, but I've done a fair bit of mods myself, including ECU remap for my current Maz3. My last car Vios can only do piggyback, which I don't want to, but now NCP93 can do reflash liao.
Dyno is when you have done at least Stage1 (IMHO), the gains won't be fantastic like ECU remap, about 3-5%?
So different strokes for different people.