Are distributions from a TSP account taxed more than the 20% paid when the distribution is taken? For instance, a $300,000.00 withdrawal is taxed at 20% when taken.

Federal tax liability depends also on your other income and deductions and credits. 

For distributions that are eligible for rollover, a minimum of 20% is required to be withheld for taxes.  By submitting Form W-4P you can request that more be withheld, but not less.  As Bsch4477 mentioned, your actual marginal tax rate on the distribution is not determined until you prepare your tax return.  It would not be unusual for your marginal tax rate on a $300,000 distribution to be more than 20%, so the default 20% tax withholding might not be enough.