I had my car dyno'd and tuned back in February. It has a naturally aspirated 3.5L V6. The tune bumped peak crank HP from 288 to 306 and torque from 254 lb-ft to 278, which is on par with the 3.7L variant of my engine.
While the peaks only gained 6 and 9%, respectively, other areas saw even bigger gains. For example, below 4000 rpm most of the power band gained 15-20 lb-ft over stock. And above 4500 rpm, the torque continues to rise rather than plateau which I've noticed in hard accelerations.
Another interesting thing is how much smoother the power band is compared to stock.
This was a bit pricey, though, and I see why most people don't do this often. All told, it was about $2500 to do: Pre-Dyno inspection, HP Tuners ECU, set of step colder spark plugs, putting it on the dyno, etc. Honestly, would do again and would like to do in the future.

I'm more familiar with ECU tuning of Hondas back in the 90s but I know it is still going on.

I had my B5 Turbo Passat chipped with no other changes. I noticed the difference during a full throttle run as the left and right front tires took turns losing grip.

Pretty sure the easy diesel remaps are over in the US but, as usual, if know someone they can set up.

The most obvious ECU tuning currently is people adding Snap. Crackle, and Pop to their cars as the engine RPMs drop.