if your running mostly state corps, the 65% tax isn't an issue, myself, i run 75%, works fine. if their was a drop in market value, given that tax wasn't adjusted, the drop is do to the over supply, as mentioned, allot of that is happening, and will continue to happen. the GM are raising production levels of all corps slowly, when that happens, many red products become green. many computer-run corps will close and open as red type. eventually it'll calm down, and return to normal, just in time for the GM to do it again.
your corps net profit affects the market value. raising tax, and profit sharing will lower this, but doesn't really effect the actual profit of the corp. if you had a drop in MV, with out adjusting taxes, then the loss was probably do to a loss of actual profit, from selling at a lower price, or not selling, as opposed to a net profit drop from raising taxes.
as far as the tax goes, higher taxes will chase off CEOs, but now-a-days, their aren't allot of 0% tax countries, you'd be safe with 20-25%. if your interested in running state corps, then the higher tax will chase away CEOs, and you can run your own corps. whats as important as tax, is your country welfare, a high country welfare >110, will give an extra boost to production, country welfare is a very important index. you can locate a graph of your country's welfare in the Social security index.
its a 6 of one or half-dozen of the other type thing. if your really successful in running corps, state corps with high tax will make you more money, if your so-so, or uncertain, then low tax and bring in CEOs.