I think the issue with HS2 is more about the project delivery, and the arrangements used.
I personally dont see why it wasn't specced for 200mph/320kph max from the begining given the distances, as any money saved would be good, but that wouldn't have avoided the massive cost overruns
Going back to original proposals, and the investigations to select the 'best' option, this is only as good as the input data, and also as with all these sort of reports can be biased towards the outcome the authors (or their paymasters) want. It suffered from the standard malaise of trying to minimise estimated costs in order to make the case, knowing full well there was no allowance for things going wrong, which they inevitably do on projects of the scale of HS2.
With the project delivery I think the issue is the government trying to shift the risk on to the contractors, which in turn creates an enviroment where every single change, even minor ones become contract variations, with all the attendent paperwork and bureaucracy. I have seen it on other major projects outside the rail industry (look at Evaporator D at Sellafield as a good example outside the rail indistry), and it ends up absorbing huge amounts of time and effort as the contractors try and 'grow' the project to cover their costs and maximise profit.