Well, there are a lot of points here! I'll have a crack at a few at least.
WW - I really can't get along with any idea of taxing anyone unless there is a truly significant benefit for the country as a whole. If it doesn't put a serious dent in the tax gap, I wouldn't support it. The money which the wealthy have is, after all, their money.
In terms of investment, I see the argument, but it could very easily get completely out of control. I don't have a great deal of confidence that the right choices (how much, which projects etc) would be made.
DD - when you quote £8bn saved through limiting pension tax relief to the basic rate, does this figure take into account that the pension on the way out should then will only attract basic rate tax, even if over the higher rate threshold? That would, in the long run, reduce the saving. The affect of pension tax relief is reducing anyway as a consequence of the caps on contributions by higher earners.
More generally, GDP isnt red herring. I agree it isn't the end of the story but historically it does seem to give a fair indication of how much we can collect in taxes, whatever taxes or rates may apply.