Fwiw I had my Fabia estate remapped, it’s the 110 version and it took it to a claimed 135bhp. It’s not made a huge difference but it has definitely improved performance, if I need to get up to speed quickly on a slip road then it’s now much easier. I’m not sure you’d notice the smaller claimed increase much on the Mazda especially given it’s an engine that needs working hard to get the best performance out if it. I don’t know if any remap companies can give an idea of likely midrange torque increases.

We have a Volvo V70, 2.4NA petrol, 140hp with a slushbox auto. It’s done 200k miles mostly in our ownership. Not once have I ever thought it needed more power.

As pointed out above my multiple people, it’s difficult to see how a naturally aspirated engine will show huge gains just from remapping. The limiting factor for power output is how much air the engine can intake. If it has an electronic throttle, it’s possible that this could be restricted, but it’s more likely that the bolt-on hardware (manifolds, throttle body, etc.) are simpler and cheaper to produce and optimized for fuel economy rather than performance. It may have a camshaft profile optimized for economy and lower-end torque, and if it has variable camshaft timing, there may be a cheap economy setup and an expensive performance setup that bolt to the same basic engine. It’s also possible that there is a different cylinder head for the cheap models and the expensive ones. You really need to check all the details before concluding that you can unlock huge gains from a remap alone.

Anyway, given the hardware that it has, it can only intake a certain quantity of air with the throttle wide open, that’s the limiting factor for peak power output. The engine management system meters fuel and adjusts spark timing to get the most efficient combustion for any combination of load and speed. Unless the factory setup is horribly bad (which is unlikely these days), there will only be marginal gains possible.

How much less mediocre are you expecting 163hp in a large family estate to be than 143hp?

Made me smile, you’re absoloutely right, but moar powah is MOAR POWAH right?

it’s difficult to see how a naturally aspirated engine will show huge gains just from remapping.

Because manufacturers detune versions to meet things like company car tax brackets, low road tax and insurance etc.

As mentioned the early-2000s NA 1.6 Mini One and Cooper were mechanically identical but the One was 90hp, the Cooper was 115. Just software.

The 60hp VW Up/Polo (and loads of derivatives) is mechanically the same engine as the 75hp version, just a different ECU.

Both can be loaded with the map of the higher output version giving big gains. Having said that, often the gain is marginal for normal driving as the extra power is at the top end of the rev range and torque increases are not as high. The Up for example has matching power/torque curves until about 5-6000 rpm so no noticeable change unless you rag it.

However, the gains on the already higher tuned NA models are much, much smaller for the reasons you highlight.