is it ethically defendable to outsource production to the developing
world where labor costs are much  lower when such actions also involve
laying off workers that work in the companys home country?

But of course it is! 

A company is not in the business of providing jobs for it's employees.
The sole purpose of any company is to produce profits for it's owners.
The owner/managers may choose to be a 'good' corporate citizen and do
many things that are good overall for not only the company's
employees, but society as a whole. However, the purpose behind any
company is to make profits, if it is more profitable to outsource
production and the company operators choose not to do so, they would
be acting against the best interests of the corporation.

there once was a weapons manufacturer who bribed politicians to start
an unnecessary war.
The consequences were horrific, thousands of times worse than 911.
But demand for their goods soared and they profited like never before.

Those manufacturers really served their "for-profit" goals well.