Flat is fine by me, but 20% is too high. Cut that in half and you're getting much warmer, IMHO.
Heres a great interview with Art Laffer, the Father of Supply-Side Economics. An excerpt:
If you tax people who work and you pay people who dont work, do not be surprised if you find a lot of people choosing not to work.
http://pittsburghlive.com/x/tribune-review/opinion/columnists/steigerwald/s_300457.html