That's a fair point, my cars are old enough to come under the older UK road tax rules so are not taxed based on emissions, but their emissions are better than the requirements for their age.

Still it would be nice if there were inexpensive ways to check emissions for your self while tinkering.

However, I think fuel maps vs. catalyst removal are largely similar, it'll either be picked up on the next inspection or VOSA can do spot checks if they think something is up. I don't think DRM / technical measures for locking the ECU are appropriate much like I wouldn't be happy if the catalyst and exhaust system were installed such that only the manufacturer could replace them.

Perhaps one would only use a performance map when using the car on a race track, it'd be a shame to replace all the electronics just for that.

I'm also not a fan of things like geo-fenced speed limiters, and replacement components that have to be coded to the car by the dealer.