Is there any long term risk with using a dump valve to either the turbo or the whole engine unit?

 

The dump valve's only function in life is to protect your turbo by dumping boost when you get off the gas (without it, closed throttle is like a brick wall to your turbo which is still spinning). Car actually drives better without it, but your turbo wont thank you.

Stock DV is recirculating - the boost 'dumped' from the pressurised intake is put back into the system before the turbo, but after the MAF (airflow) sensor. This maintains correct fuelling.

Aftermarket DV's are either recirc or 'dump to atmosphere' - I'm not aware of any long term problems caused by fuelling innacuracies introduced by 'dump to atmosphere' valves, but pops, bangs and flames are said to be introduced if you're decatted.

I'm going no further...try a search

Richard