> current withholding rate is substantially under most engineers' marginal tax bracket

RSUs are withheld at Federal bonus rates, which seemed pretty reasonable (it's 25% until your total taxable comp from an employer hits $1M in the tax year, and then it jumps to 39.6%); and California equity statutory rates, which is the second to top marginal rate, which works out right, unless you made enough to pay the mental health tax.

Although, given that state tax isn't deductible anymore, perhaps 25% isn't nearly enough at the federal level anymore?