Yes, percentage of total income paid is a better metric. But then there is the issue you raised about the difficulty of quantifying state income and sales taxes. I suspect, but do not have the data to verify one way or another, that the "upper middle" income region is where the highest percentage of income is paid, and that the "wealthy" pay considerably less as a percentage of their income, due to loopholes, the lower capital gains rate, and the regressiveness of sales and payroll taxes. However, until we have all the information we can't really say for sure.
I'm against including SS taxes in the issue. Under tax theory I do not believe it an actual 'income tax'. IMO, it's more of a forced contribution to a retirement plan and set of insurance policies (e.g., disability).
I wouldn't include sales tax or r/e tax either, neither are relevant in a discussion of national (income) tax policy since neither are national taxes, and are used for purposes different from (national) income tax. For similar reasons I would exclude state level income tax. States are not bound by the constitutional requirement that taxes be "proportional", thus they have revenue options the federal govt does not and this, to some extent, may unusually influence their income tax policy.
I think we'd find that many people making a million or more $'s actually do pay a very high rate. Musicians/recording artists, prof athletes, actors, TV personalities, highly commissioned sales people, and top level execs who either don't receive stock options, or whose stock options had no value all pay at the highest rate.
However, and assuming they invest smart, eventually their overall rate may be diluted due to the lower rate on LT cap gains they get on investment income and dividends. (ST w/b taxed at 35%). However, they will stay pay the maximum on their main income (music royalties, salary for athletes and actors etc.)
Aside from hedge fund managers, we will have a small population of the uber wealthy who may not work but exist only to collect investment income. This is mostly inherited money, such as the Gores and Hiltons. Originally, the estate tax was designed to address this group but Congress, being what it is, has screwed it up and made it possible to avoid (tax avoidance is perfectly legal).
LT capital gains, as we presently calculate them, should NOT be taxed as regular income. Inflation is not income. Assets held over a long period will carry substantial amounts of inflation. The answer is simply to adjust for inflation, then tax the real gain at ordinary (high) rates. Again, this fault lies with Congress. IMO, there is no reason a person who holds an asset for 366 days should get the same break, ostensibly for inflation, as a person who held their asset for 20 yrs.
There are simply to few CPA's in Congress IMO.
Fern