By joining the startup with its startup salary, you've already thrown away tens if not hundreds of thousands of dollars of hard cash you could have earned by taking that Google offer instead. So if you feel that this is a rational decision, spending another $10k to minimize a lot of tax pain down the road is also entirely rational.

It's also an interesting quirk of human psychology that we value the loss of something we have (here $10k in cash) so much more than the cost of not having something in the first place.