Context: I have a Ford Taurus AWD with a naturally aspirated 3.5L V6 (288 hp & 254 lb-ft, stock). It had reached the point in life where it’s old enough that I’m ok tinkering with it, but new enough that it’s still worth tinkering. There was also zero info about tuning this engine and I wanted to fix that. So I brought it to a dyno.
I went with an HP Tuners ECU. The only other “mod” to my car was a set of one-step colder spark plugs; everything else is stock. Edit: I also stuck with 87 octane.

The shop managed to increase the peak wheel horsepower from 217 hp to 231; and the torque from 203 lb-ft to 222. Increases of 14 hp and 19 lb-ft, or 6% and 9%, respectively. Assuming the parasitic losses is the same, my new power at the crank is roughly 306 hp and 278 lb-ft. Pretty similar to the stock 3.7L that Ford made.
But as you can see from the graph, other areas improved even more. Some sections received 20+ lb-ft, and 15+ hp above 5000 rpm.
Mileage seems to have improved, too. However I still need to test that more. I’ve only had this tune for a week now.

I haven’t done many hard accelerations, but it does feel a little faster when accelerating. Pre-tune, I remember feeling a surge in power around 4000 rpm; I’m glad the dyno proved me right.
In terms of feel, the most noticeable difference is in city driving. The engine feels so much more responsive when I start to press on the pedal from a standstill. But not in a jerky, aggressive way (unless I choose to floor it); it’s still a very comfortable daily driver. It just “feels” more put together, if that makes sense.

Total cost for the dyno and tune was around $1500-2000. Plus any extraneous repairs they required me to do before I could go on the dyno (I.E. I had a leaky transaxle seal that cost $525 to replace). So, I see why not many people choose to get their cars tuned. At the same time, I feel this was worth it for the science.
If anyone has any questions, I’ll try to answer them as best I can.