I can guarantee you that there are some notable counter examples to your theory. I can tell you that manufacturers (BMW, VW,...) found out that it's a lot cheap

I can guarantee you that there are some notable counter examples to your theory. I can tell you that manufacturers (BMW, VW,...) found out that it's a lot cheaper to manufacture the same physical engine and just limit it in software for different 'levels'. BMW E90 series for example, from around 2008 onwards the 116d and 118d have exactly the same engine (per BMW part numbers mind you) but a different power output, even 120d only has one part difference (the ones with N47 engine that is). On VW part, new-er VW Caddy and VW Transporter with diesel engine option(s) have the same physical engine (in the model year) with widely different power outputs -> because they differentiate it for different segments via software. And even moving from that, because of regional rules in NL for example, the cars with up to 140HP are prelevant and ofcourse VAG is not making a new engine just for NL, they limit one of their exsisting ones. So yes, I believe that cars like this can be safely tuned without _any_ significant unwanted consequences for the longeviety of the engine/car. Mostly because the maps/factors can be adapted from their siblings with more factory power. Regarding the black smoke/soot - no I believe that this is a (very) bad tune or something physicaly wrong with the car. It could be that they removed the DPF/EGR for some reason or something like that. But this is a whole other can of worms.