Jake is on the money there Alex, however the phenomenon is a little more complex than that ! When a turbo shaft breaks- due to over speeding, the seals get damaged and allow oil to pass on the exhaust- very smoky, or in the intake- which will lead to engine runaway and bent rods. The 1.9Tdi has a very quick spooling turbo which, by removing restriction and poor control of the N75 will allow the shaft to break at it's weakest point. Just remember it's simple physics- cold side is compressing air and it's slowing down and the hot side is still driven by the exhaust flow. The harmonic differences (aka surge) is what places huge strain on the compressor shaft and sometimes the compressor blades do deform as well. Spool down surge is not as bad but the best option when tuning is to fit a larger turbo. The extra mass on the turbine will be better for building boost gradually but won't feel as snappy as the gt17 at producing boost, unless you go for a later gen. GTB1756vk which has a better hot side and bigger compressor so you can have best of both worlds.
In pd130 case with a remap is better to leave the cat in place to smooth surges out IF you keep the gt1749Va turbo, if you go larger then by all means, get rid of it !