Im quite willing to be educated and it would be nice if an expert would chime in. I have read about overspeed. I havent had it properly explained. I am not as techie as I used to be but I have spent a fair bit of time twiddling turbos when you could do it with spanners. The speed of the turbine is controlled by the wastegate plus engine rpm. Less back pressure = more turbine speed. Thats just what we want, less lag. In fact if we could, we could make it instant. At low rpm it isnt going to overboost as it isnt spinning fast enough. As rpm builds turbine speed increases as does boost. Wastgate (VNT in our case) controls boost and hence rpm of turbine. It is possible to get overboost spikes but that usually throws codes cuts fuel and all sorts.
In the old days we used to crank up the wastegates to increase boost. Car goes faster untill you hit some kind of limit usually electronic boost limit in ecu. That was all we could have untill some bright spark reverse engineered an ecu and removed this limit. Then we could have whatever boost we wanted untill something else broke, that was how all this chipping business started. I managed to blow off all the hoses on a cosworth once, think it was over 4bar.
AW If its any use when I bought my car at 58k it had codes stored for blocked dpf bank1. It drove great with no other signs. I deleted the code and had months trouble free. At 68k one problem after another and as I have said I can force regen myself but decided to delete dpf. For your average non enthusiast its probably best left alone.
Anyone please feel free to chime in...