The Laffer curve is not interesting when you are way on the left side of it.
If high government debt was decreasing economic participation that would be cause for concern. But is there any reason to believe it is? Japan is running a much higher debt-to-GDP ratio and while people are (probably rightly) worried, the fact that nothing catastrophic has happened there yet suggests that there's at least that much slack available.
The question is whether tax revenues could be raised by raising tax rates to the point that revenues exceeded current expenditure. Whether or not that is possible is uncertain.