GM's stock price has very much to do with GM's actions. GM can do countless things which will impact its equity value.
Also, changing the effective|marginal tax rate by 50% has material implications on your business model and capital structure, e.g. debt (and tax shields).
10% to 20% is a 100% increase not a 50% increase. Now sure, paperwork changes, but the price of your widget or the layout of the factory etc don't really care about the taxes outside of extreme cases.
Anyway, profitable companies like GM can issue stock to raise capital or buy back stock. Buybacks don't really depend on the stock price as it's not an investment it's another form of dividend. Issuing stock is an inefficient way to raise capital better to issue bonds or not issue dividends.
Now sure, there are second order effects of stock price such as stock options. But again +/- 10% to stock price on a given day does not do much.
PS: Consider Microsoft if the tax rate where to increase to say 40% what would they change?
...and 20 to 10 is 50%. I chose the smaller % as it's still substantial.
For your widget company, it does matter! ;-) What widgets you make, your price, your price relative to competition, market share protection, pricing power, where you build your factory, PP&E decisions, and more all depend on your tax rates. I promise, and want to compete against firms that overlook these parameters.
For GM, you're overlooking other (mis)management decisions that will show up in the share price.
Right now, MSFT is facing just such an issue re: re-patriating money from overseas. If there were a one time foreign tax holiday as floated by Obama, MSFT would choose very different decisions than the status quo in terms of buy backs and R&D spend. In terms of operations changing, I guarantee they'd re-think their debt and their product mix within their 3 reporting segments. Some products wouldn't be profitable enough to sell if the profit margin shifted 2-3%.
I don't nessisarily disagree with you examples. But, I think you are misssing the forest for the trees.
I have had several successful CEO's all tell me to ignore taxes. That does not mean you install or don't install solar panels based on tax breaks. It can be very important, but it's generally premature optimization. Further, it's not the top tax rate that is important it's differential tax rates aka A @X or B @ less than X.
MSFT's re-patriating money is a good problem to have. They may see a tax holiday in the next administration or they may not. But again, it's getting to that point is the issue.
Microsoft (and other international companies) go to great lengths to optimize and defer taxes into the future. As the US already has among the highest corporate tax rates in the world not much would change as all international revenue is being held overseas anyhow. There would be changes in how they allocate their resources but only a CFO might be able to tell you what those would be.
GM's stock price has very much to do with GM's actions. GM can do countless things which will impact its equity value.
Also, changing the effective|marginal tax rate by 50% has material implications on your business model and capital structure, e.g. debt (and tax shields).