I think the issue would be that he would not be paying on his current amount. In other words he's paying on his income after all these write offs and crap. Under 9-9-9 I believe he would be paying on all income without any cuts or contributions. Buffet said in 2010 he paid 7 million in taxes, but under 9.9.9 he'd have to pay on the billions from berkshire hathaway he was protected from under current law. I think the problem you have is you are counting his income based on current tax practices, but his income would be counted differently under the new system.
Like for example I make say 300 million a year under IRS rules but my taxable income after everything is 50 million, so I end up paying 7 million income tax, which is about 15% ish. But under 9.9.9 i pay 9% of 300 million not the 50 million adjusted down with all these deductions and crap. So now Buffet ends up paying the neighborhood of 27 million. But the result would be buffet would have no reason to donate 23 million like he did in 2010 to charity to get tax write offs. In the end instead of paying 7 million in 2010 for IRS and 23 million for deductions and paying over 30 million towards society he'd be paying 27 million straight to uncle sam.
The impact on middle class would also be confusing, for example a family making 60,000 a year might have a very low tax liability due to child tax credits and stuff like that, but under this play you don't get that. So taxes go way up, but you don't pay FICA, so the result would actually be more IRS tax but less overall fica/irs combined tax.