Try 600+ for the salary index. You get 55 to 60% back in income taxes. The raw materials cost of plutonium is so high that 125% production will pay for most of the extra salary.
If you increase the corporations production through salaries or welfare, then it will produce more. But the corporations estimated production will always be configured to 100% production (unless it has been changed recently), when selling to the world market or offering to CM. Anything above 100% production level will only ever be referred to the world market for this reason.
Also sheep your countries income tax rate will decrease with better finances (and maybe population and employment). When I first start playing SC the income tax rate was about 60%, now its 20%.