From conservative, Bruce Bartlett, former budget director for Ronald Reagan:
What I think is indisputable is that we could raise the top rate quite a bit [to at least 50% tax rate] before any significant Laffer Curve effect kicked in. I would also add that the main effect insofar as revenues are concerned has to do with legal tax avoidance and changing the composition of income rather than having rich people just stop working or investing. It's pretty obvious from the experience of other countries that rich people will pay very considerably more taxes than they do here before they withdraw from the labor force.
For a series of takes on this question, see Ezra Klein's round-up of opinions, which tend toward 60-70% with a few oddities at 20%.
Even at 50%, that is quite a bit more than today's top rate.