The unforseen consequence of this is that people in danger of breaching the threshold have less incentive to work. At the moment, anyone taxed at 40% obtains tax relief on anything they put into a pension plan. Once they reach the ~$1MM threshold that tax relief is no longer available, and some of those people will decide to take early retirement. Unfortunately, many of these people are quite useful to society (which is how they come to be earning those sums). For example I know a GP in his 40s who works a 3-day week because he'd rather spend the other 2 days pursuing his hobbies than paying tax at a marginal rate of 40%. This is someone who will have cost the country a lot to train, is undoubtedly doing an important job, but the amount of work he does is effectively throttled by taxation policy.
As with any tax raid, not only do you have to figure out what rate will maximise revenue (see Laffer Curve) but also what the impact will be on society.