I've had a couple of diesel cars mapped at SP Tuning near Hinckley in Leicestershire:
https://www.sptuning.co.uk/index.html
I've had no issues at all with these, including the cars going back to main dealers (1 VW & 1 Audi) for servicing and being hooked up to their computers with the remap obviously not being visible to them.
It really does work. The maps that manufacturers use are inevitably a compromise between emissions, performance, fuel grade etc. etc. If a more focused map is used then I have found improvements in both performance AND economy (no doubt at the expense of emissions and/or ability to run on low grade fuel).
Obviously there are boundaries, if the boost pressure and fuelling are turned up to '11' then it's not going to do the vehicle any good. But if you use a reputable company that knows what they're doing and keeps power increases to a sensible level then you shouldn't have any problems.